Hello there!
A subject complement is the adjective, noun, or pronoun that follows a linking verb. (most know linking verbs: be, become, and seem)
In this case, we identy the linking verb of the sentence.
The new park in the neighborhood has been busy all week.
The linking verb is be, conjugated in the present perfect: has been, and what is next to it? the word busy. Taking into account that its function is to give an attribute to the suject, what kind of word is "busy"?
A. noun? It is not a noun because there is no a determinant before it.
C. pronoun? as it is not the suject of the sentence (because it is "the new park = it), it is not a pronoun.
Therefore, correct answer: B. Adjective. It is an adjective because you are complementing the pronoun with an information that qualifies it: an attribute.
Hope this helps
