B is the answer to your question
I believe the linking verb is “will”. It’s linking the subject “the United States of America” and the predicate “be 300 years old!”
The best anser is C - gaunt. Gaunt means thin and looking exhausted, such as when one is old or tired.
The answer can be actually found directly in the poem, which contains the line: 'An aged thrush, frail, gaunt, and small'. Gaunt is used directly in the poem!
The sentence in which the underlined infinitive phrase functions as a subject is the last one - to quickly move large groups of people is the purpose of public transportation.
<em>To quickly move </em>is the infinitive phrase, and it is a subject.