The sentence "What _______ you do if you had more free time?" is a second conditional sentence.

The sentence "What _______ you do if you had more free time?" is a second conditional sentence.

The structure of a second conditional is:
If + past simple, would + base form of the verb.

In this sentence, the "if" clause is "if you had more free time" (past simple). Therefore, the main clause requires "would" followed by the base form of the verb.

The correct option is:
would