Here's why:
* Simple Subject: The main noun or pronoun that performs the action of the verb.
* "This kind of argument" acts as a single unit describing the argument, but it's not the actual thing performing the action.
* "Hard" is an adjective describing the argument, not the subject of the sentence.
* "To answer" is an infinitive phrase that modifies the subject, but is not the subject itself.
Therefore, the simple subject is "argument" because it is the noun that is the subject of the verb "is" (implied in the sentence).