De or het reserve-eenheid?

The correct Dutch article is:

de reserve-eenheid

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

How to use reserve-eenheid

the reserve-eenheidde reserve-eenheid
this reserve-eenheiddeze reserve-eenheid
that reserve-eenheiddie reserve-eenheid
our reserve-eenheidonze reserve-eenheid
your reserve-eenheidjouw reserve-eenheid
every reserve-eenheidelke reserve-eenheid
which reserve-eenheid?welke reserve-eenheid?
a big reserve-eenheid / the big reserve-eenheideen grote reserve-eenheid, de grote reserve-eenheid