C. IS
It is the linking verb in the sentence, This storm is worse than the last one.
Linking verbs are verbs that links the subject to the rest of the sentence. It is also called a copula or copular verb.
These are all linking verbs: be (am, is, are, was, were, has been, are being, might have been), become, and seem.
There are also some action verbs that functions as linking verbs.