C. Mark studied for his math test with Margie. This is the correct option.
"To study" is the infinitive and when this is used in the past, the final "y" turns into "i" and then the suffix "ed" is added.
These options are wrong:
A. Mark is studying for his math test with Margie. This is an instance of present continuous and the verb "to study" is in the present participle.
B. Mark will study for his math test with Margie. This is an instance of simple future, so the verb " to study" is in the bare infinitive.