De or het stapelring?

The correct Dutch article is:

de stapelring

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

This word is not in the word lists we use to double-check answers.

How to use stapelring

the stapelringde stapelring
this stapelringdeze stapelring
that stapelringdie stapelring
our stapelringonze stapelring
your stapelringjouw stapelring
every stapelringelke stapelring
which stapelring?welke stapelring?
a big stapelring / the big stapelringeen grote stapelring, de grote stapelring