In the life cycle of Plasmodium, the protozoan parasite that causes malaria in humans, select the four statements that are true from the following list: A) Inside an infected mosquito, haploid gametes fuse, forming a diploid zygote that develops into an oocyst. B) Plasmodium cells infect two specific types of cells in humans. C) Inside an infected mosquito, haploid gametes fuse, forming a haploid zygote that develops into an oocyst. D) When a mosquito bites a human with malaria, diploid Plasmodium cells enter the mosquito. E) Plasmodium cells are transferred to humans through mosquito saliva. F) The life cycle of Plasmodium involves only haploid forms. G) The complete life cycle of Plasmodium requires two hosts.

Introduction / Context:
Plasmodium is the protozoan parasite responsible for malaria, a serious infectious disease that involves both human and mosquito hosts. Its life cycle is complex and alternates between sexual stages in the mosquito and asexual stages in the human body. This question asks you to apply your understanding of the Plasmodium life cycle by identifying which four given statements are true. To answer correctly, you must know where sexual reproduction occurs, which human cells are infected, how the parasite is transmitted, and the nature of its haploid and diploid stages.

Given Data / Assumptions:
- We are dealing with the typical Plasmodium life cycle that causes human malaria.
- Sexual reproduction of the parasite takes place in the Anopheles mosquito, not in humans.
- In humans, Plasmodium infects specific cell types rather than all tissues.
- Transmission from mosquito to human occurs when the mosquito injects saliva during a bite.
- The parasite alternates between haploid and diploid stages and requires two different hosts to complete its life cycle.

Concept / Approach:
To solve this, evaluate each statement against standard biological facts. Haploid gametes fuse in the mosquito gut to form a diploid zygote, which then develops into an oocyst that releases many haploid sporozoites. In humans, Plasmodium primarily infects liver cells and red blood cells. Transmission to humans occurs via sporozoites in mosquito saliva, and the entire life cycle needs both a human host and a mosquito host. Any statement that contradicts these key ideas must be rejected as false.

Step-by-Step Solution:
Step 1: Check statement A. It correctly describes fusion of haploid gametes in the mosquito forming a diploid zygote that develops into an oocyst, so A is true. Step 2: Check statement B. In humans, Plasmodium infects liver cells first and then red blood cells, which are two specific cell types, so B is true. Step 3: Check statement C. It incorrectly calls the zygote haploid, but by definition a zygote is diploid, so C is false. Step 4: Check statement D. The mosquito ingests gametocytes, which are haploid, not diploid, so this statement is false. Step 5: Check statement E. Sporozoites are injected into humans through mosquito saliva during a bite, so E is true. Step 6: Check statement F. The life cycle includes both haploid and diploid stages, so saying it involves only haploid forms is false. Step 7: Check statement G. The full life cycle indeed requires both a human host and a mosquito host, so G is true. Step 8: Combine the true statements, which are A, B, E, and G, matching option A.

Verification / Alternative check:
An alternative way to verify is to remember that humans are the intermediate host where asexual reproduction occurs, and mosquitoes are the definitive host where sexual reproduction occurs. Only in the mosquito does a diploid zygote stage appear. Also, remember the classic order: sporozoites (mosquito to human), liver stage, blood stage, gametocytes (human to mosquito), and back to sexual stages in the mosquito. This overview confirms that statements involving diploid zygotes in mosquitoes, infection of liver and red blood cells, transmission by saliva, and a two host life cycle are correct together.

Why Other Options Are Wrong:
Option B combines several incorrect statements and omits key true ones, so it does not match the correct set of four true statements.
Option C uses combinations that rely on C and F, which are individually wrong because they misdescribe the ploidy of the zygote and ignore the diploid stage.
Option D claims that none of the given combinations are correct, which is incorrect because A, B, E, and G form a valid and accurate set.
Option E includes D and F, both of which are false according to standard descriptions of the Plasmodium life cycle.

Common Pitfalls:
Students often confuse haploid and diploid stages and may think the life cycle is entirely haploid. Another common error is assuming that sexual reproduction occurs in humans because that is where clinical symptoms appear. Some learners also forget that Plasmodium has a very specific host range and cell tropism, which is why it infects only certain cell types in humans. Careless reading of the statements can lead to accepting partially correct descriptions that have a small but important error in ploidy or host location.

Final Answer:
The correct combination of true statements is A, B, E, and G.