De or het proefboring?

The correct Dutch article is:

de proefboring

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

How to use proefboring

the proefboringde proefboring
this proefboringdeze proefboring
that proefboringdie proefboring
our proefboringonze proefboring
your proefboringjouw proefboring
every proefboringelke proefboring
which proefboring?welke proefboring?
a big proefboring / the big proefboringeen grote proefboring, de grote proefboring