Final answer:
The sentence 'Mia is planning to audition for the school play next spring' is correctly completed with the infinitive phrase 'to audition for the school play', representing option D.
Step-by-step explanation:
The correct answer choice that completes the sentence with an infinitive phrase is D. to audition for the school play.
An infinitive phrase will begin with 'to' followed by the simple form of the verb. In this case, 'to audition' is the infinitive phrase that correctly completes the sentence: "Mia is planning to audition for the school play next spring."