De or het studentendemonstratie?

The correct Dutch article is:

de studentendemonstratie

Rule Words ending in -tie and -sie: almost always de. See the rule

How to use studentendemonstratie

the studentendemonstratiede studentendemonstratie
this studentendemonstratiedeze studentendemonstratie
that studentendemonstratiedie studentendemonstratie
our studentendemonstratieonze studentendemonstratie
your studentendemonstratiejouw studentendemonstratie
every studentendemonstratieelke studentendemonstratie
which studentendemonstratie?welke studentendemonstratie?
a big studentendemonstratie / the big studentendemonstratieeen grote studentendemonstratie, de grote studentendemonstratie