No rule for this one
No ending or word group predicts the article of App. Learn it as a pair and say it out loud: die App.
die App
It's die App. App is feminine, plural die Apps.
Welche App benutzt du?
Which app do you use?
Wir entwickeln eine App.
We're developing an app.
No ending or word group predicts the article of App. Learn it as a pair and say it out loud: die App.