De or het ecologisme?

The correct Dutch article is:

het ecologisme

Rule Words ending in -isme: almost always het. See the rule
Meaning (Dutch) (politiek, filosofie) ideologie die de ecologie voorop stelt

How to use ecologisme

the ecologismehet ecologisme
this ecologismedit ecologisme
that ecologismedat ecologisme
our ecologismeons ecologisme
your ecologismejouw ecologisme
every ecologismeelk ecologisme
which ecologisme?welk ecologisme?
a big ecologisme / the big ecologismeeen groot ecologisme, het grote ecologisme

Meaning, plural, diminutive, pronunciation and translations: WikiWoordenboek (Dutch Wiktionary) (CC BY-SA 4.0). Example sentences: Tatoeba (CC BY 2.0 FR).