Emphatic stress is the placement of greater stress on a particular word in a phrase or sentence. In this case, the word wife is emphasized to correct a mistake that has been made in the question. To find the appropriate question to the response: I bought my WIFE a new dress, you have to find the question that has a word that is different from the emphasized word/ the word in capital letters. The correct question is

A. Did I buy my husband a new dress?

No. I bought my WIFE a new dress.