💡 Detailed Explanation
The correct choice is 'とても' (very / extremely). The adverb '非常に' (hijō ni) expresses an extreme degree, which is highly synonymous with 'とても' (totemo). Sentence translation: 'This summer is extremely/very hot, isn't it?' '少し' means a little, 'まあまあ' means so-so, and 'ぜんぜん' means not at all.