Explanation:

The correct answer is option 'C', which is "took".

To understand why 'took' is the correct answer, let's break down the sentence and analyze the different options:

- Taken: This is the past participle form of the verb 'take'. It is used in passive voice constructions or as an adjective. However, in the given sentence, we need a verb form that functions as the subject of the sentence.
- Taking: This is the present participle form of the verb 'take'. It is used to show an ongoing action. However, in the given sentence, we need a past tense verb to indicate that the action has already been completed.
- Took: This is the past tense form of the verb 'take'. It is used to indicate an action that occurred in the past. In the given sentence, the action of taking the final exam has already happened, so 'took' is the appropriate verb form.

Therefore, the correct answer is 'took' because it is the past tense form of the verb 'take' and correctly indicates that the students have already completed the action of taking the final exam.

In summary:
- The verb form 'took' is the correct answer because it is the past tense form of the verb 'take'.
- 'Taken' is a past participle form and is not suitable for use as the subject of the sentence.
- 'Taking' is a present participle form and indicates an ongoing action, which is not appropriate in this context.