Which sentence uses dashes correctly?

a.
we had one thing in mind: swimming—in the ocean.

b.
the phone—just kept ringing and ringing all afternoon.

c.
the music teacher gave him a choice of horns—for example, the trumpet, the tuba, or the saxophone.

d.
a red chair sat in front of—the fireplace.

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.