Are the contestants on Jeopardy! still required to respond in the form of a question?

Hello everyone. Last week, I was watching Jeopardy! with my parents and we noticed that one of the contestants did not always say, “What is/who is [blank]?” Previously, my parents and grandmother have told me that if you don’t answer in the form of a question, it does not count. However, last week, Alex Trebek accepted the answer without saying who or what is. Have the rules changed or something? I’ve tried going online too and couldn’t find a good answer. Could someone please help me? Thank you.