Is it b?
is the underlined word a predicate nominative or a predicate adjective?
those bananas might still be (( but they are becoming mushy.
a. predicate nominative
b. predicate adjective

A predicate nominative is the first noun following a linking verb such as the verb to be in all of its forms. A predicate adjective is the first adjective after it. Having this in mind, as well as the fact that the word tasty is an adjective, and not a noun, the correct answer is indeed B. predicate adjective, you are right. :)