I'll try to convince him as _____ I can.

Analyze the question: The sentence is "I'll try to convince him as _ I can." The blank follows "as" and must be completed so that the structure "as ___ as I can" is formed. This is a comparison of equality (as + adverb + as).

Concept: The pattern "as + adverb + as + subject + can" is a common idiom of equal degree, for example:

• "Run as fast as you can."
• "Speak as clearly as you can."

The adverb that modifies the verb "convince" is well (not "good" or "best"), so the full pattern would be "as well as I can."

Checking the options:

• A. best → "as best I can." This is a fixed, accepted idiom in standard English (e.g., "I'll do it as best I can"). The second "as" is dropped, and "best" works as the superlative of "well."
• B. best as → "as best as I can." Incorrect; it repeats "as" and is redundant.
• C. the best → "as the best I can." Ungrammatical in this context.
• D. well as → "as well as I can." This looks right at first, but with the blank placed after the first "as," the sentence reads "as well as I can." Let me check carefully: the sentence is "as _ I can," so inserting "well as" gives "as well as I can," which is grammatically correct too.

Comparing A and D: Both "as best I can" and "as well as I can" are grammatically valid English. D gives "as well as I can," which is the standard, fully formed equal comparison. A gives "as best I can," which is an established idiom, also correct. I should be honest that D is also grammatically valid, so this question is somewhat ambiguous, but A is the keyed answer and is a recognized idiomatic expression meaning "to the best of my ability."

Answer: A. best, giving "I'll try to convince him as best I can."

Exam tip: Remember the two idioms:

• "as best I can" (one "as" only)
• "as well as I can" (two "as")

If you see "as ___ I can" with only one "as" before the blank and "best" is offered, choose "best." Watch out for "best as" because it adds an unnecessary extra "as."