Answer:
D. No, because it is neither defensible nor arguable.
Explanation:
A counterclaim is supposed to provide a convincing argument and enough evidence to prove a claim wrong.
The reason Juan's friend presents to prove Juan wrong only demonstrates his own ignorance towards the subject.
The counterclaim is not defensible since not having experience on a topic is no excuse to assert it is of no relevance, and it is not arguable because it can be proven that in fact he has never taken a class on the subject - although that doesn't make the counterclaim any valid.