Final answer:
The word 'durable' in the sentence functions as a predicate adjective, modifying the subject 'materials' and linked to it by the linking verb 'must be'.
Step-by-step explanation:
The word in bold, durable, functions as a predicate adjective in the sentence provided. A predicate adjective follows a linking verb and modifies the subject of the sentence. In this case, 'durable' is describing the 'materials,' which is the subject, and is linked by the verb 'must be,' which is a linking verb. Therefore, the correct answer to the student's question is option b) Predicate adjective.
Following the review guidelines, we can identify that there is no direct object in this sentence since the verb 'must be' is not a transitive verb. Instead, it functions to connect the subject, 'materials,' with the complement, 'durable.' The main action or state of being in the sentence is carried by the linking verb 'must be,' which links the subject to the adjective that describes it.