De or het diagnosestelling?

The correct Dutch article is:

de diagnosestelling

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

How to use diagnosestelling

the diagnosestellingde diagnosestelling
this diagnosestellingdeze diagnosestelling
that diagnosestellingdie diagnosestelling
our diagnosestellingonze diagnosestelling
your diagnosestellingjouw diagnosestelling
every diagnosestellingelke diagnosestelling
which diagnosestelling?welke diagnosestelling?
a big diagnosestelling / the big diagnosestellingeen grote diagnosestelling, de grote diagnosestelling