Mary..... her homework at the moment A)does B)doingC)is doingD) do

Mary is doing her homework at the moment. В истиннее время Мария делает семейное задание.

 Is doing is in Present Continuous. We use Present Continuous for action that takes place at the moment. Present Continuous is formed with the help of auxiliary to be. The present forms of to be I am, you are, he is, she is, we are they are.

We add the suffix ing to the verb.

I am doing my homework.

Negative is formed with the help of not auxiliary + not + verb + ing.

Interrogative is formed to be + subject + verb + ing.  

Если деяние происходит в данный момент (at the moment), означает это время Present Continuous.
Оно создается маршрутом написания глагола to be (формы для местоимений) и добавления к глаголу окончания -ing.
Решение задания:
Ответ: с).