Read the sentence. To function properly, the phone needed to be charged. Which correctly identifies the infinitive phrase? To function properly the phone needed to be charged the phone needed to be charged.

The correct answer is "to function properly"

This is an infinitive phrase that consists of the infinitive "to function", and the adverb that serves as a modifier that is "properly". To be charged could also be treated as an infinitival phrase but here it is in its passive form and doesn't have the same function as the "to function properly" phrase.