C) serving to identify; distinguishing
In the sentence, the word distinctive is used to describe the three ways the Peachtree Road Race is unique. Option A is wrong because ordinary or typical is the opposite of unique and the goal is to show how the race is anything but ordinary and typical. Extremely difficult is also not a good option because the traits listed do not necessarily show that the race is difficult. Option D is just wordy and wrong. Option C is the best option because the things that are described as distinctive are the things that identify the race as unique.
