The italicized word "rain" belongs to the class of verb in structural linguistics.
The verbs in a sentence are the action words that describe what the subject is doing. Verbs, which describe what is happening, are the primary component of a sentence or phrase together with nouns.
The main verb is also known as the lexical verb. The main verb in a sentence—the one that often expresses the action or state of the subject—is referred to by this word.
The main verb "rain" in the sentence is denoting an action that is going to happen in the future.
Hence, the correct option is D that is "verb".
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