No rule for this one
No ending or word group predicts the article of Obst. Learn it as a pair and say it out loud: das Obst.
das Obst
It's das Obst. Obst is neuter.
Ich esse jeden Tag Obst.
I eat fruit every day.
No ending or word group predicts the article of Obst. Learn it as a pair and say it out loud: das Obst.