De or het klepopening?

The correct Dutch article is:

de klepopening

Rule Words ending in -ing: almost always de. See the rule

How to use klepopening

the klepopeningde klepopening
this klepopeningdeze klepopening
that klepopeningdie klepopening
our klepopeningonze klepopening
your klepopeningjouw klepopening
every klepopeningelke klepopening
which klepopening?welke klepopening?
a big klepopening / the big klepopeningeen grote klepopening, de grote klepopening