De or het vetmesting?

The correct Dutch article is:

de vetmesting

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

How to use vetmesting

the vetmestingde vetmesting
this vetmestingdeze vetmesting
that vetmestingdie vetmesting
our vetmestingonze vetmesting
your vetmestingjouw vetmesting
every vetmestingelke vetmesting
which vetmesting?welke vetmesting?
a big vetmesting / the big vetmestingeen grote vetmesting, de grote vetmesting