De or het investeringsplanning?

The correct Dutch article is:

de investeringsplanning

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

How to use investeringsplanning

the investeringsplanningde investeringsplanning
this investeringsplanningdeze investeringsplanning
that investeringsplanningdie investeringsplanning
our investeringsplanningonze investeringsplanning
your investeringsplanningjouw investeringsplanning
every investeringsplanningelke investeringsplanning
which investeringsplanning?welke investeringsplanning?
a big investeringsplanning / the big investeringsplanningeen grote investeringsplanning, de grote investeringsplanning