He used to pray everyday, believing that as long as he . . . . . . . . so, he would be safe.

is doing
does
did
will do

The correct answer is C. did.

The sentence is in the past tense, so the verb must also be in the past tense. The other options are not in the past tense.

A. is doing is in the present tense.
B. does is in the present tense.
D. will do is in the future tense.

The sentence is about a habit that the person used to do in the past. The person no longer prays every day.