When_____________your new dishwasher?a) you bought b)did you bought c) did you

When did you buy your new dishwasher?

We cant use You bought because we need an auxiliary verb after interrogative word.

We cant use did you bought and do you bought because we use infinitive with an auxiliary, the auxiliary takes the past tense. (to do did) The auxiliary verb is did. Did is the second form of the irregular verb to do. The question is Special question and the question is in Past Indefinite. We use Past Indefinite with when.