Final answer:
The sentence contains an B) infinitive phrase, 'to hear', which functions as an adverb modifying the verb 'was'. The infinitive is the base form of a verb with 'to' preceding it, and it can act in various roles such as a noun, adjective, or adverb.
Step-by-step explanation:
The sentence "She was sad to hear that she had hurt their feelings" contains an infinitive phrase. The infinitive phrase here is to hear, which is the base form of the verb hear preceded by the word to. This infinitive phrase functions adverbially, providing a reason for her sadness by modifying the verb 'was'. In general, an infinitive can act as a noun, adjective, or adverb in a sentence. For example, in the sentence "A fistfight is no way to resolve an argument.", the infinitive phrase to resolve an argument acts as an adjective modifying the noun way.