Answer:
The infinitive phrase is "To do one's best" and the object of the infinitive phrase is "best".
Step-by-step explanation:
An infinitive phrase starts with an infinitive - "to" and the simple form of a verb. It also includes objects and or modifiers. For example:
- To achieve my goals is a necessity I have in life. - the underlined section is the infinitive phrase.
The object of an infinitive phrase is the word (noun or pronoun) being affected by the infinitive. In the example above, "my goals" is the object.
Having that information in mind, let's analyze the sentence given in the question:
- To do one's best is the ultimate goal. - the underlined section (infinitive + object) is the infinitive sentence.
The word being affected by the infinitive is "best", which makes it the object - it's as if we are answering the question "to do one's what?"