1. Where were you doing then?
- I was taking a shower.
2. What were you doing then?
- I was taking a shower.
(Which question is right?)
#2 (both question and answer) is correct. #1 is incorrect.
Both questions are grammatically correct and can be used interchangeably. The only difference is the use of "where" and "what" which alters the focus of the question.
Question 1: "Where were you doing then?" implies that the person asking wants to know the location or place where the action of taking a shower was happening. For example, it could be asked if the person was taking a shower at home, at a gym, or at a friend's house.
Question 2: "What were you doing then?" focuses on the action itself, asking for an explanation of what the person was engaging in at that specific time. In this case, the action is taking a shower.
Both questions are correct, but they emphasize different aspects of the situation.