The answer is D) The cost of conquering and defending a Western empire were too great.
Justinian tried to reconquer the former Roman Empire territories. He was very successful at it first. The Byzantine armies conquered the North Africa and Sardinia.
He was eager to get more victories and sent his army to Italy. But the Ostrogoths were strong too and fighting continued for years. Even Franks joined the fight and won a few battles.
But the overall effects were devastating. The population of Italy reduced significantly, the armies perished from fighting, and even Justinian captured Rome at some point, his armies could not defend the city against invaders. It was just too costly, and hard to defend.