Correct! The answer here is c.
There are a couple of things going on here. First, the less wordy of the choices is the right one in this case, as it often is on these tests. In the first possible answer, why use eleven words and then use the word "this" for the noun to work with the word "is"? Why not just turn the beginning part into a noun in the first place to link with "is"? Well, that's what is done in answer c. The nine words in answer c beginning with the word "Crossing" is actually a noun (-ing verb forms are actually nouns called gerunds). (By the way, any sentence answer that uses "the fact that" like choice d does will never be a right answer--there's always a better way or shorter way to say something without using that phrase.) The skills below should help you understand long noun forms better.