The simple future is a verb tense that’s used to talk about things that haven’t happened yet, here, raining. We use the simple future to talk about an action or condition that will begin and end in the future. The formula for the simple future in the negative is will + not + [root form of verb], that is, will not rain or won't rain. Hence, Option B is correct.
The rest of the options do not express this condition. They either mention actions that have already happened (options C and D) or actions that are happening in the present (option A).
