All of the options listed could be related to a composer's work in some way, but the most direct job a composer could have among these choices is:
A conductor for an orchestra
Conducting often involves interpreting and leading performances of a composer's work, which is closely related to the role of a composer. However, a composer could also write for commercials, perform in a choir, or be an accompanist, but those roles are less directly tied to the act of composition itself.