Human beings enjoy music, and the taste is universal across cultures. Why is that pair of facts not yet enough to establish that a taste for music is an adaptation?

B. Because universality is consistent with the trait being a by-product.

Universality is weaker evidence than it looks, because a rival explanation predicts it just as well: pitch discrimination, temporal prediction and emotional response all have their own reasons for existing, none of them musical. The question is never whether a trait is useful or common, but whether variation in it affected fitness through the claimed function.