__ - Mary at school today?" "No, she was very sick and stayed home. a. Was b. Does c. Did d. Were

The correct answer is c. Did.

In this sentence, the question is asking about an event that happened in the past, specifically whether Mary was at school today. The verb "did" is used to form questions in the past tense, so the correct choice is "Did." The sentence would then read: "Did Mary go to school today?"