💡 Detailed Explanation
The correct choice is 'なかなか上手' (quite/fairly good). When used in an affirmative sentence, the adverb 'なかなか' (nakanaka) means 'quite', 'considerably', or 'beyond expectations', carrying a positive nuance similar to 'とても' (very). Sentence translation: 'He is quite good at the piano.' 'だいぶ' means 'considerably' but is less idiomatic here for praise. 'ぜんぜん' requires a negative verb. 'たいてい' means 'mostly/usually'.