Answer:
D: Before his European travel -- the highlight of his summer -- Mickey forced himself to pay off his credit cards.
Step-by-step explanation:
The correct use of em dashes is shown in the sentence of the option D, which uses one of the functions of em dashes: the one of replacing commas for an appositive phrase. In this case, "the highlight of his summer" is an appositive, and presents additional information about the noun "travel"; it could have been written with commas. But the em dashes are more emphatic.