Explanation:
Explanation:
This sentence is an example of a third conditional, which is used to describe a hypothetical situation in the past — something that didn’t happen, but we’re imagining what the outcome would have been if it had.

Structure of the Third Conditional:
If + past perfect, would have + past participle

Example:

If he had left earlier, he would have caught the train.

Breaking down the sentence:
"If this had happened to you..." → this is the past perfect tense

The second clause must match with "would have + past participle"

So: "What would you have done?" is the correct match.

Why the other options are incorrect:
A. would you do – This is used for present/future hypotheticals, not past.

B. have you done – Present perfect, doesn’t fit the past conditional.

D. will you do – Future simple, not appropriate for a past condition.

E. would you be doing – This is a continuous conditional form and doesn’t fit the structure of a third conditional.