いい

行(い)かなく ても いい です よ。

The correct answer is "いい". This question tests the N4 grammar pattern 「～なくてもいいです」 (nakute mo ii desu), meaning "do not have to do ~" or "it's okay not to do ~". The structure is the negative TE-form (～なくて) + も + いいです. In this sentence, 「行かなくてもいいです」 means "You don't have to go". "いい" is essential to complete this common phrase, indicating that something is "good" or "acceptable". "行かなく" is incorrect because it's a negative stem and needs "ても" after it. "ても" is already present before the blank. "です" is a polite copula that would come after "いい", but it's not the missing word to complete the core grammar pattern.

正解は「いい」です。「～なくてもいいです」は「～しなくても大丈夫だ」という意味のN4文法です。「行かなくてもいいです」で「行かなくても大丈夫です」となります。「行かなく」と「です」は文脈に合いません。「ても」はすでにあります。