De or het polstering?

The correct Dutch article is:

de polstering

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

How to use polstering

the polsteringde polstering
this polsteringdeze polstering
that polsteringdie polstering
our polsteringonze polstering
your polsteringjouw polstering
every polsteringelke polstering
which polstering?welke polstering?
a big polstering / the big polsteringeen grote polstering, de grote polstering