Tomorrow, if rain falls, I will not go out. [Syntax]

The correct answer is "降る" (furu). This sentence uses the conditional pattern "~なら" (if ~). The verb that connects to "なら" must be in its plain form. "雨(あめ)が降(ふ)る" (ame ga furu) is a plain form clause meaning "rain falls." Therefore, "明日(あした)は雨(あめ)が降(ふ)るなら、出(で)かけません" correctly translates to "Tomorrow, if it rains, I will not go out." "雨(あめ)" and "が" are already in the sentence, forming the subject. "なら、" is the conditional conjunction that follows the verb, not fills the blank before it.

正解は「降る」です。「〜なら」は動詞の普通形に接続し、「もし〜なら」という条件を表します。「雨が降るなら」で「もし雨が降るならば」という意味になります。「雨」と「が」はすでに文中にあり、「なら、」は動詞の後ろにくるため不適当です。