The blank links a subject (new variety) and an object (heat). Thus, we need a verb. We can strike out b and d, which are not verbs.
Excellent. Let's take a look at the two verbs of a and c. The difference between these two is that one refers to a singular subject, and the other refers to a plural one. What is the subject in this sentence (that the ... verb is referring to?) It is "new variety", or rather "variety". This is a singular noun (even though variety refers to more than one thing. Be careful!) Thus, since we have a singular noun, we need to use the singular verb which is a) tolerates.
