De or het roommengsel?

The correct Dutch article is:

het roommengsel

Rule Words ending in -sel: usually het. See the rule

How to use roommengsel

the roommengselhet roommengsel
this roommengseldit roommengsel
that roommengseldat roommengsel
our roommengselons roommengsel
your roommengseljouw roommengsel
every roommengselelk roommengsel
which roommengsel?welk roommengsel?
a big roommengsel / the big roommengseleen groot roommengsel, het grote roommengsel