No rule for this one
No ending or word group predicts the article of Mensch. Learn it as a pair and say it out loud: der Mensch.
der Mensch
It's der Mensch. Mensch is masculine, plural die Menschen.
Er ist ein netter Mensch.
He is a nice person.
No ending or word group predicts the article of Mensch. Learn it as a pair and say it out loud: der Mensch.