De or het koopbeslissing?

The correct Dutch article is:

de koopbeslissing

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

How to use koopbeslissing

the koopbeslissingde koopbeslissing
this koopbeslissingdeze koopbeslissing
that koopbeslissingdie koopbeslissing
our koopbeslissingonze koopbeslissing
your koopbeslissingjouw koopbeslissing
every koopbeslissingelke koopbeslissing
which koopbeslissing?welke koopbeslissing?
a big koopbeslissing / the big koopbeslissingeen grote koopbeslissing, de grote koopbeslissing

Dutch words ending in koopbeslissing

aankoopbeslissing

In compound words, the last part decides the article. See the rule