The correct answer is D. misplaced modifier. A misplaced modifier is a word or a phrase that is located in the wrong part of a sentence - meaning that it makes an error of being separated from the word or a phrase it is supposed to modify, or describe. Here, the misplaced modifier is with an incredible sound system, because the way this sentence is written, it would seem this modifier refers to the summer vacation, and not to the convertible Mustang.
