Answer:
The correct answer is "To run a marathon"
Explanation:
An infinitive phrase is formed by the preposition "to" + the bare verb+object.
Infinitive phrases can act as nouns, adjectives and adverbs.
In this example, the infinitive phrase is functioning as an adverb since it is explaining "why" Natalyn is training.
The rest of the options are not infinitive phrases.
Option A is formed by subject+verb in past tense
Option B is formed by present continous
And option D is formed by the nouns "coach" and "nutricionist"
