Final answer:
The underlined phrase is a participial phrase in the sentence. It begins with a present participle and functions as an adjective modifying the subject.
Step-by-step explanation:
The underlined phrase in the sentence "Lying in bed, I heard a strange noise." is a participial phrase. A participial phrase is a phrase that begins with a present or past participle, which is a verb form that ends in -ing or -ed. In this sentence, the present participle "lying" is used to describe the action of the subject (I) and functions as an adjective modifying the subject.
Learn more about Identifying types of phrases in sentences