Mary Clarissa Miller

There's one crucial piece of information missing here. Agatha Christie was born Agatha Mary Clarissa Miller.

This could have been a genuine mistake, or maybe the setter thought the question would have been too easy if the first name had been included. If it was the former, then I can't believe none of the vetters stopped to think where the name Agatha came from; if the latter, well IMHO this is a classic example of being economical with the truth (and I think we all know what that's a euphemism for).

If you think a question's too easy, just ask a different question.