The correct answer would be alternative B) "She spent four days trying to put together the perfect outfit. In the end, she settled for something simple: a black tee, ripped jeans, and a headband."
According to the dictionary, the colon (:) is a punctuation mark used to start a list of items, a quotation or an explanation. In this case, it's used to start a list of items, describing the outfit chosen by the girl.
On the other alternatives, it took the place of other punctuation marks, and none of them were used to start any of the possibilities shown above.
