De or het fluoridering?

The correct Dutch article is:

de fluoridering

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

How to use fluoridering

the fluorideringde fluoridering
this fluorideringdeze fluoridering
that fluorideringdie fluoridering
our fluorideringonze fluoridering
your fluorideringjouw fluoridering
every fluorideringelke fluoridering
which fluoridering?welke fluoridering?
a big fluoridering / the big fluorideringeen grote fluoridering, de grote fluoridering