Answer: The most sensible answer would be C.
Explanation: If D was the answer, how would chocolate house become so popular if only the wealthiest people could afford it? That doesn't make sense, so D cancels out. B is just a restatement of the second fact "the Spanish began adding sugar to chocolate water" but doesn't make sense because why would people adding sugar to chocolate water in Central America cause chocolate houses to become popular in Europe? Spain is in Europe, not Central America. That cancels out. A makes some sense but C is a stronger answer and ties all the facts together. Most people do not like regular chocolate because it is so bitter, which is why most people go for semi-sweet or milk chocolate which has been sweetened to taste, therefore making it more enjoyable.