A 'wh' question is asked for information about something.
The given sentence says how heavily 'it was raining'.
Option A: This is the most suitable question, as it asks "how was it raining", it wants information on how it was raining (scarcely or heavily).
Hence option A is correct.
Option B: This is a yes/no question. It does not ask for any information but asks if it was raining or not. Hence this isn't an appropriate question.
Option C: This is also a yes/no question that does not ask for information (about how it rained). It requires a confirmation - 'Yes, it was'.
Hence options B and C are incorrect.
Option D is incorrect because the answer lies in A.