Answer:
4. Should be studying
Explanation:
The predicate is the part of a sentence (or clause) that contains a verb, stating what the subject is doing, or describes the subject.
When we have a main verb and an auxiliary verb accompanying it, both of them together form the predicate. In the given sentence, we have a construction with a main verb to study and an auxiliary verb should: should be studying. If you chose just studying or should be as the predicate, that would be incorrect as those two options are not the complete predicate.
