In the following sentence with a blank, choose the correct word to complete the definition: An animal species becomes _____ when its last existing member dies.

Introduction / Context:
This question combines basic science with English vocabulary. It asks for the correct word that completes a standard definition used in biology and environmental studies. Understanding this term is important for topics related to conservation, biodiversity, and ecology, as well as for general knowledge sections in competitive examinations.

Given Data / Assumptions:

• Sentence: An animal species becomes _____ when its last existing member dies.• Options: exist, existing, extinct, extract.• The sentence clearly aims to define what happens to a species when there are no living individuals left.• We assume the usual scientific terminology used in school textbooks.

Concept / Approach:
In biology, the term extinct is used for a species when there are no surviving individuals anywhere in the world. The definition essentially reads: A species is considered extinct when the last existing member has died. The other options either are verbs or unrelated words and do not complete the definition meaningfully. We need an adjective that describes the state of the whole species after the final death.

Step-by-Step Solution:
Step 1: Identify that the blank requires a state or condition of the animal species, so an adjective is needed.Step 2: Examine extinct, which is an adjective meaning no longer in existence as a living species.Step 3: Check exist, which is a verb and does not fit grammatically after becomes in this specific definition.Step 4: Look at existing, which is a present participle or adjective that normally describes things that currently exist, the opposite of what the sentence means.Step 5: Consider extract, which is a completely unrelated verb or noun referring to removal or a concentrated substance.Step 6: Conclude that extinct is the only option that correctly names the condition of the species when the last individual has died.

Verification / Alternative check:
Recall textbook definitions, which usually state that a species is said to be extinct when there is no surviving member alive on earth. If we insert extinct into the sentence, we get An animal species becomes extinct when its last existing member dies, which exactly matches that standard wording. None of the other options produce a scientifically accepted definition. For instance, becomes exist and becomes extract are clearly incorrect English, and becomes existing has the opposite meaning.

Why Other Options Are Wrong:
exist is a verb meaning to be present or alive, and would require a different grammatical structure such as continues to exist or ceases to exist. existing is an adjective that describes something that is still there, not something that has disappeared. extract has no connection to the status of populations or species and generally refers to taking something out or a concentrated product like vanilla extract. These words cannot express the idea that a species has completely died out.

Common Pitfalls:
Most candidates familiar with basic science will immediately recognise extinct, but some may be distracted by existing because of the presence of the word existing member earlier in the sentence. It is important to see that the first use of existing describes the last individual, while the blank refers to the new status of the entire species after that individual has died. Keeping track of which part of the sentence each word refers to helps avoid confusion.

Final Answer:
The correct word to complete the definition is extinct.