De or het landontginning?

The correct Dutch article is:

de landontginning

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

How to use landontginning

the landontginningde landontginning
this landontginningdeze landontginning
that landontginningdie landontginning
our landontginningonze landontginning
your landontginningjouw landontginning
every landontginningelke landontginning
which landontginning?welke landontginning?
a big landontginning / the big landontginningeen grote landontginning, de grote landontginning