De or het slipvrij?

The correct Dutch article is:

de slipvrij

Rule Words ending in -ij and -erij: usually de. See the rule

How to use slipvrij

the slipvrijde slipvrij
this slipvrijdeze slipvrij
that slipvrijdie slipvrij
our slipvrijonze slipvrij
your slipvrijjouw slipvrij
every slipvrijelke slipvrij
which slipvrij?welke slipvrij?
a big slipvrij / the big slipvrijeen grote slipvrij, de grote slipvrij