De or het monsterfusie?

The correct Dutch article is:

de monsterfusie

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

How to use monsterfusie

the monsterfusiede monsterfusie
this monsterfusiedeze monsterfusie
that monsterfusiedie monsterfusie
our monsterfusieonze monsterfusie
your monsterfusiejouw monsterfusie
every monsterfusieelke monsterfusie
which monsterfusie?welke monsterfusie?
a big monsterfusie / the big monsterfusieeen grote monsterfusie, de grote monsterfusie