(A) What matters to the argument is that composers were loners, not necessarily why they were loners. That has no bearing on the argument, which is that intimate relationships produce happiness, period.

(B) It is mentioned that composers spent time in solitude and had no intimate relationships. These are mentioned as two separate qualities.

(C) Solitude may or may not be necessary for composition, but either way the argument does not change.

(D) This again may or may not be true, and the argument would not change. We care about the well known composers.

(E) And here's the obvious assumption. If the greatest composers were not happy, then the theory would be confirmed. So the conclusion, that the theory is wrong, can only be valid if the composers were happy.