Answer:
Most of the salt is gone, too.
Step-by-step explanation:
The chosen sentence matches an singular noun phrase like Most of the salt with the singular form of the verb to be, is. As a result, there is a parallel structure, in which the number of the noun corresponds with the number of the verb.
On the other hand, the other option contains a plural noun phrase like Most of the stars and a singular verb, is, so it lacks parallelism. Thus, the correct version of it is:
Most of the stars are hidden.