No rule for this one
No ending or word group predicts the article of Frau. Learn it as a pair and say it out loud: die Frau.
die Frau
It's die Frau. Frau is feminine, plural die Frauen.
Frau Becker ist meine Lehrerin.
Ms Becker is my teacher.
Das ist meine Frau Sara.
This is my wife Sara.
No ending or word group predicts the article of Frau. Learn it as a pair and say it out loud: die Frau.