De or het thematentoonstelling?

The correct Dutch article is:

de thematentoonstelling

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

How to use thematentoonstelling

the thematentoonstellingde thematentoonstelling
this thematentoonstellingdeze thematentoonstelling
that thematentoonstellingdie thematentoonstelling
our thematentoonstellingonze thematentoonstelling
your thematentoonstellingjouw thematentoonstelling
every thematentoonstellingelke thematentoonstelling
which thematentoonstelling?welke thematentoonstelling?
a big thematentoonstelling / the big thematentoonstellingeen grote thematentoonstelling, de grote thematentoonstelling