An infinitive phrase is demonstrated as the phrase that comprises of an infinitive i.e. 'to plus the basic form of the verb' followed by any of the modifiers or compliments. These phrases can function as adjectives, adverbs, or noun in the sentence.
In the given sentence, the infinitive phrase would be 'to go on a field trip' as it begins with infinitive('to plus the basic form of verb 'go') followed by the compliment 'on a field trip' and it is functioning as an adjective to describe the noun 'the teacher's plan'. Therefore, <u>option A</u> is the correct answer.
