Final answer:
The subject of the question is a phrase.
Step-by-step explanation:
A phrase is a group of words that does not contain a subject and a predicate, while a clause is a group of words that contains a subject and a predicate and can stand alone as a sentence.
In the statement 'One of the most significant speeches in American history is this,' the phrase 'One of the most significant speeches in American history' serves as the subject of the sentence. Therefore, the phrase 'One of the most significant speeches in American history' is the subject of the question.