The correct option for the sentence "The exercise must be done .... possible" is C. as quickly as. Let's break down why this is the correct choice and analyze the other options. Step-by-Step Explanation

1. Understanding the Sentence Structure:
2. The phrase "must be done" indicates that we are discussing the manner in which the exercise should be completed. The word "possible" suggests that we are looking for a way to express the idea of doing something in the best or most efficient way.
3. Identifying the Correct Adverb:
4. The word "quickly" is an adverb that describes how the exercise should be done. Adverbs modify verbs, and in this case, "quickly" modifies "done."
5. Evaluating the Options:
6. A. as quick as: This option uses "quick," which is an adjective. In this context, we need an adverb to describe the action of doing the exercise. Therefore, this option is grammatically incorrect.
7. B. as quicker as: This option is incorrect for two reasons. First, "quicker" is a comparative form of the adjective "quick," and it should not be used with "as...as" construction. The correct form would be "as quick as" or "quicker than," but not "as quicker as." This makes this option grammatically incorrect.
8. C. as quickly as: This is the correct option. "Quickly" is the adverb form of "quick," and it fits perfectly in the sentence structure. It conveys the idea that the exercise should be done in the quickest manner possible.
9. D. as most quick as: This option is also incorrect. "Most" is used to form the superlative degree, which is not appropriate in this context. The correct superlative form would be "the quickest," but that does not fit the "as...as" structure.
10. Conclusion:
11. The only grammatically correct and contextually appropriate option is C. as quickly as. It effectively communicates that the exercise should be completed in the fastest manner possible.

Summary of Why Other Options Are Incorrect:

• A. as quick as: Uses an adjective instead of an adverb.
• B. as quicker as: Incorrect comparative form; should not use "as" with "quicker."
• D. as most quick as: Incorrect use of "most" for a comparative structure; should use "the quickest" instead.

Revision Summary:

• The correct answer is C. as quickly as.
• "Quickly" is the adverb form needed to modify the verb "done."
• Adjectives (like "quick") cannot be used in this context where an adverb is required.
• Avoid using incorrect comparative forms or superlatives in "as...as" constructions.