De or het co-educatie?

The correct Dutch article is:

de co-educatie

Rule Words ending in -tie and -sie: almost always de. See the rule
Meaning (Dutch) gemeenschappelijke opvoeding van en gemeenschappelijk onderwijs aan leden van beide seksen

How to use co-educatie

the co-educatiede co-educatie
this co-educatiedeze co-educatie
that co-educatiedie co-educatie
our co-educatieonze co-educatie
your co-educatiejouw co-educatie
every co-educatieelke co-educatie
which co-educatie?welke co-educatie?
a big co-educatie / the big co-educatieeen grote co-educatie, de grote co-educatie

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