Fill in the blank with the correct option: I tried to help him best of my ability.

The correct answer is B. The fixed expression is "to the best of one's ability", meaning as well as one is able to. The sentence reads: "I tried to help him to the best of my ability."

Why the others fail:

• at: "at best" means in the most favourable case ("the plan is useful at best"). It does not combine with "the best of my ability".
• from: there is no standard collocation "from the best of my ability". "From" does not fit this expression.
• of: "of" appears in phrases like "the best of friends" or "the best of times", but the ability phrase needs "to" before "the best".

The core concept is idiomatic collocation. Many English prepositions are locked into set phrases that cannot be worked out by logic. Such items recur in the English section of PCS prelims, usually as fill-in-the-blanks and idiom sets. Learn them as whole chunks, not as isolated words. The same precision with fixed expressions also sharpens your descriptive writing for Mains.

Trap: Aspirants pick "at" because they remember "at best" and "at its best". Remember the full chunk "to the best of my ability/knowledge" as one unit.