The summary of what happened is right here: https://www.sparknotes.com/lit/gatsby/section7/
The rule about subjects joined by 'and' is that there must be a subject-verb agreement. This means that if the subject is singular, then the verb must be also singular. On the other hand, if the subject is plural that means the verb must also be plural.
The Answer is: It is their only way out.
"Yes," replied the Professor, looking at me calmly from under his spectacles, "it is the only chance which remains to us of ever escaping from the interior of the earth to the light of day."
The correct option should be option c. If referring to the squiggly line under a misspelled word.