The correct answer is The phrase acts as a noun that is the subject of the
sentence.
Step-by-step explanation:
In the text, the section "volunteering at the animal shelter" is a phrase because this includes multiple words that go together. Moreover, this phrase is a noun because it refers to a specific activity, and the grammatical function in the sentence is subject. This is because the activity "volunteering at the animal shelter" is the one that precedes the verb "has been" as it occurs with most subjects and it is the activity being described. Therefore, these words refer to the agent of the action or subject.