Question

Phil was training for the marathon, so he had to __ rain from the junk food.

A. Main
B. Foun
C. Con
D. Abs

Answer 1

The correct answer is D. abstain

Answer 2

Assuming you need to merge the prefixes with -tain, the correct answer is D. abs.
The verb to abstain means to restrain yourself from doing something, in this case, from eating junk food. Maintain means to continue something, fountain is a piece of architecture, and contain means to have something in something else. So only D makes sense.