Answer: it rains
Explanation: In English grammar, when using future time clauses with words like “when,” “as soon as,” “unless,” “until,” and “in case,” the verb in the clause following these conjunctions is in the present indefinite tense. The main clause, however, is in the future indefinite tense.

In the sentence above, ‘in case’ and the preceding clause are in the future indefinite tense, so the part that follows should be in the present indefinite tense, making the correct answer ‘it rains.’