De or het referendumverordening?

The correct Dutch article is:

de referendumverordening

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

How to use referendumverordening

the referendumverordeningde referendumverordening
this referendumverordeningdeze referendumverordening
that referendumverordeningdie referendumverordening
our referendumverordeningonze referendumverordening
your referendumverordeningjouw referendumverordening
every referendumverordeningelke referendumverordening
which referendumverordening?welke referendumverordening?
a big referendumverordening / the big referendumverordeningeen grote referendumverordening, de grote referendumverordening