De or het stemvastheid?

The correct Dutch article is:

de stemvastheid

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

How to use stemvastheid

the stemvastheidde stemvastheid
this stemvastheiddeze stemvastheid
that stemvastheiddie stemvastheid
our stemvastheidonze stemvastheid
your stemvastheidjouw stemvastheid
every stemvastheidelke stemvastheid
which stemvastheid?welke stemvastheid?
a big stemvastheid / the big stemvastheideen grote stemvastheid, de grote stemvastheid