In this English grammar error detection question, identify which part of the sentence "Photosynthesis is possible because of sun" contains an error.

Introduction / Context:
This sentence combines a scientific term, photosynthesis, with a simple grammatical structure. The scientific idea is correct, but the language contains a small error that exam setters want you to notice. The sentence reads Photosynthesis is possible because of sun. The task is to find which part of the sentence is not grammatically acceptable in standard English and would need correction in formal writing.

Given Data / Assumptions:
- The full sentence is: Photosynthesis is possible because of sun. - It is divided into parts as follows: Part 1 Photosynthesis is, Part 2 possible, Part 3 because of sun, and Part 4 No error. - Only one part contains an error. - The scientific fact, that photosynthesis depends on the sun, is assumed to be correct.

Concept / Approach:
The error in this sentence is not about science but about the use of articles in English. When we refer to the sun as the unique star of our solar system, we normally use the definite article the, as in the sun. The phrase because of sun is therefore incomplete. The correct form should be because of the sun. We must identify which numbered part contains this incorrect phrase.

Step-by-Step Solution:
Step 1: Examine part 1: Photosynthesis is. This is a correct subject verb combination introducing the topic. Step 2: Examine part 2: possible. This adjective correctly completes the verb phrase is possible. Step 3: Examine part 3: because of sun. Here we see the missing article the before sun. Step 4: Understand that unique objects in nature, such as the sun, the moon, and the earth, usually take the definite article. Step 5: Part 4 claims no error, but we have identified one, so part 4 cannot be correct. Step 6: Conclude that part 3 is the error containing segment.

Verification / Alternative check:
We can rewrite the sentence correctly as Photosynthesis is possible because of the sun. This version sounds natural and follows the standard article rule. A second check is to compare with similar expressions: Life on earth is possible because of the sun. The earth orbits the sun. In all such sentences, we use the definite article. Using only sun without the article would sound incomplete or incorrect in formal written English. This confirms that the error is located in part 3, and the correction is to add the before sun.

Why Other Options Are Wrong:
Option A is wrong as an answer because Photosynthesis is is a correct opening and does not contain an error. Option B is wrong because the adjective possible is correctly placed after the linking verb is. Option D is wrong because the sentence is not error free; it requires a correction in part 3.

Common Pitfalls:
Students may become distracted by the scientific term photosynthesis and look for problems in that word, but the exam is concerned with English grammar, not science. Another common mistake is to overlook small words like articles because they seem minor. In reality, articles are a frequent source of errors in competitive exams. Training yourself to check for the, a, and an whenever there is a singular countable noun or a unique object can significantly improve your accuracy in these questions.

Final Answer:
The correct answer is Part 3: "because of sun."