De or het sluiering?

The correct Dutch article is:

de sluiering

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

How to use sluiering

the sluieringde sluiering
this sluieringdeze sluiering
that sluieringdie sluiering
our sluieringonze sluiering
your sluieringjouw sluiering
every sluieringelke sluiering
which sluiering?welke sluiering?
a big sluiering / the big sluieringeen grote sluiering, de grote sluiering

Dutch words ending in sluiering

ontsluiering

In compound words, the last part decides the article. See the rule