Answered

The Western Church claimed authority over the Eastern Church because the Pope?

Awanted the Eastern Church to remain Christian.
Bwanted the Eastern Church to move to Rome
Cbelieved he had power over both churches
Dbelieved he had power over European kings.

Verified Answer :

The western Church claimed authority over the Eastern Church because the pope believed he had power over both Churches.

Answer:

(just to it a lil easier from the answer above.) The answer is indeed C.

"believed he had power over both churches."

I got it right on edge 2020.