Final answer:
The sentence with an adverbial phrase is "Sarah will start her presentation in an hour," as "in an hour" is a phrase that modifies the verb and doesn't contain a subject and a verb.
Step-by-step explanation:
The sentence that contains an adverbial phrase, as opposed to an adverbial clause, among the provided options is "Sarah will start her presentation in an hour."
An adverbial phrase is a group of words that act as an adverb in the sentence without having a subject and a verb, as opposed to an adverbial clause, which does have a subject and a verb. In this sentence, "in an hour" modifies the verb "will start" by telling us when Sarah will start her presentation. It is not a clause because it does not contain a subject-verb combination.
On the other hand, sentences 1, 3, and 4 contain adverbial clauses. For instance, in sentence 1 - "Don't believe him if he says he's telling the truth," the clause "if he says he's telling the truth" has a subject "he" and a verb "says" and functions as an adverb, specifying a condition for not believing him.
The sentence that contains an adverbial phrase, as opposed to an adverbial clause, is number 2 - "Sarah will start her presentation in an hour."
An adverbial phrase is a group of words that functions as an adverb, providing additional information about the verb. In this sentence, the phrase "in an hour" modifies the verb "start" by indicating when Sarah will start her presentation.