De or het mondeling?

The correct Dutch article is:

het mondeling

Exception Words ending in -ing usually take de, but mondeling is an exception. See the rule

How to use mondeling

the mondelinghet mondeling
this mondelingdit mondeling
that mondelingdat mondeling
our mondelingons mondeling
your mondelingjouw mondeling
every mondelingelk mondeling
which mondeling?welk mondeling?
a big mondelingeen groot mondeling
the big mondelinghet grote mondeling