Answer:
Most of the salt is gone, too.
Explanation:
The chosen sentence matches an singular noun phrase like <em>Most of the salt</em> with the singular form of the verb to be, <em>is</em>. 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 <em>Most of the stars</em> and a singular verb, <em>is</em>, so it lacks parallelism. Thus, the correct version of it is:
<em>Most of the stars </em><em>are </em><em>hidden.</em>
