The correct option is B What
Pronouns that express surprise or disbelief are exclamatory pronouns and require an exclamation mark (!) at the end of the statement. 'What' is one such exclamatory pronoun. The given sentence expresses surprise at the downpour that took place the evening before, and 'what' is the most suitable option, being an exclamatory pronoun. Hence option B is correct. 'Which' and 'who' are interrogative pronouns, and don't express surprise. They're not exclamatory, hence options A and C are incorrect. 'Whenever' also expresses surprise or confusion, but it is an adverb that's used in questions. It's used for emphasis instead of 'when' in questions and is not exclamatory, hence option D is incorrect.