* Infinitive phrases start with the word "to" followed by a verb (in this case, "work"). They act as nouns, adjectives, or adverbs.
Let's break down why the other options are incorrect:
* A. verb: While "made" and "work" are verbs, the entire phrase "to work harder" acts as a unit.
* B. prepositional: Prepositional phrases start with a preposition (like "in", "on", "at"). There isn't a preposition in this phrase.
* C. gerund: Gerunds are verbs ending in "-ing" that act as nouns. "Working" would be a gerund, but the phrase has "to work".
Therefore, "to work harder" is an infinitive phrase because it functions as a noun within the sentence.