De or het vetsmering?

The correct Dutch article is:

de vetsmering

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

How to use vetsmering

the vetsmeringde vetsmering
this vetsmeringdeze vetsmering
that vetsmeringdie vetsmering
our vetsmeringonze vetsmering
your vetsmeringjouw vetsmering
every vetsmeringelke vetsmering
which vetsmering?welke vetsmering?
a big vetsmering / the big vetsmeringeen grote vetsmering, de grote vetsmering