the answer is C, because a dash is used to "indicate" an aside to the point you are making, which, in this case, is that you should'nt wear sandals or sneakers on a hike :)
Explanation : The sentence A is incorrect because there should be two dots : (we want to explain how was the grand canyon).
the sentence B is incorrect because it's an hyphen, it's here to join two words or parts of words together.
as for the last sentence, it's also incorrect because of the hyphen.
Hope it helps !