Answer:
Correct answer is B. sailing around the Horn of Africa.
Step-by-step explanation:
B is the correct answer because Portuguese sailors and explorers started expeditions to prove this theory. The first one to try to do it was Bartolomeu Dias, but his expedition failed due to strong winds and tide. Still, couple of years later Vasco de Gama proved this theory to be correct as he sailed around the Horn to reach Calcutta.
A is wrong because Europeans didn't known much about Africa south of Sahara and weren't convinced that that could be possible.