Final answer:
The phrase 'should last about two hours' in the sentence is an adverbial phrase that modifies the verb and provides information about the duration of the movie.
Step-by-step explanation:
In the sentence 'The movie should last about two hours,' the phrase 'should last about two hours' functions as an adverbial phrase. According to the sentence patterns that occur with the be verb, when a form of be serves as the main verb of a sentence, it can be followed by an adverbial of time or place, an adjectival, or a noun phrase. However, in this sentence, 'should last' is not a form of the verb 'be', but it similarly operates to describe the verb 'last', providing information about the duration of the movie, which is a characteristic of an adverbial phrase.
An adverbial phrase modifies a verb in the sentence to provide additional information about time, place, manner, condition, frequency, or degree. In this example, 'about two hours' describes the expected duration of the movie, which is essentially indicating the 'how long' aspect pertinent to the action of the verb 'last'. Therefore, 'should last about two hours' acts as an adverbial phrase modifying 'last'. The correct option is adverb.