The answer is the option C. The music teacher gave him a choice of horns
- for example, the trumpet, the tuba or the saxophone, because it is
correctly separating two main clauses; in this case the second clause
contains and explanation of the first one. In the options A, B and C the
dashes are splitting phrases that should not be split.
Honestly, all of them can. I would select them all. Context is the only one that’s iffy, but context is also important because words can have connotative meanings that alter their meaning and tone.