De or het heideontginning?

The correct Dutch article is:

de heideontginning

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

How to use heideontginning

the heideontginningde heideontginning
this heideontginningdeze heideontginning
that heideontginningdie heideontginning
our heideontginningonze heideontginning
your heideontginningjouw heideontginning
every heideontginningelke heideontginning
which heideontginning?welke heideontginning?
a big heideontginning / the big heideontginningeen grote heideontginning, de grote heideontginning