The correct option is D When / What about
The correct answer is Option D.
WHEN is used when we want to about time. In the given sentence, the speaker wants to know the time of rain. WHEN, therefore completes the first sentence.
WHAT ABOUT is used when we ask about something. Usually it is used there is continuity in the discussion. In the first sentence, the speaker wants to know of rain and in the continuity of discussion he wants to know of snow. Therefore, WHAT ABOUT completes the second sentence.
Hence Option D is the correct answer.
The remaining options are incorrect because:
Option A: WHY is used to ask reason, which is not the relevant in the context of the sentence.
Option B: WHY/HOW are used to ask reason and manner respectively. This irrelevant and incorrect in the given context.
Option C: WHAT TIME/WHO - WHAT TIME is used to ask the exact time. In the first sentence, the speaker wishes to know the time of rain in general. WHO is used to ask about a person.