Select the best word for the blank in the following sentence.
The medicine that is responsible for curing a host of deadly diseases _________ in dangerously low supply.

A subject must agree with its verb in number. This means both need to be singular or both need to be plural. The subject in this sentence (the medicine) is singular so we need a singular verb (is).
The trap here is that the subject (the medicine) is separated from the verb by a plural noun (diseases). To avoid this trap, simply find the subject and the verb and make sure they agree. Ignore everything else.