De or het nonnenkleding?

The correct Dutch article is:

de nonnenkleding

Rule Words ending in -ing: almost always de. See the rule
Meaning (Dutch) of kloosterkleed genoemd, is de specifieke dracht die door kloosterzusters (nonnen) wordt gedragen. Het is een symbool van hun toewijding aan God, armoede, kuisheid en gehoorzaamheid

How to use nonnenkleding

the nonnenkledingde nonnenkleding
this nonnenkledingdeze nonnenkleding
that nonnenkledingdie nonnenkleding
our nonnenkledingonze nonnenkleding
your nonnenkledingjouw nonnenkleding
every nonnenkledingelke nonnenkleding
which nonnenkleding?welke nonnenkleding?
a big nonnenkleding / the big nonnenkledingeen grote nonnenkleding, de grote nonnenkleding

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