True the war in Europe came to an end on May 8, 1945.
One of the main ways in which Spain benefited from the 1494 Treaty of Tordesillas was that it clarified that certain lands in the New World that were in dispute (since Portugal had also claimed them previously) would go to Spain, thus enlarging the empire.
Due to the deaths of most of the population, "ordinary farm workers, who had been forced into serfdom previously, shot up by 50%. More bullion among a smaller population meant more wealth across all classes and because the same land was in use and it was now plentiful, it resulted in technological advances as well." - Hank Campbell (found at Science20.com)
About a third of the clergy fell to the Black Death, making answer a. unlikely.
I never found any information stating Peasants migrated into cities for medical care, making answer b. unlikely.
Lower classes benefited from the Black Death economically, therefore c. is my final answer.
Doctors at this time knew nothing about how to cure disease, they also made little to none advancements. Answer d. is very wrong.