De or het haarverlenging?

The correct Dutch article is:

de haarverlenging

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

How to use haarverlenging

the haarverlengingde haarverlenging
this haarverlengingdeze haarverlenging
that haarverlengingdie haarverlenging
our haarverlengingonze haarverlenging
your haarverlengingjouw haarverlenging
every haarverlengingelke haarverlenging
which haarverlenging?welke haarverlenging?
a big haarverlenging / the big haarverlengingeen grote haarverlenging, de grote haarverlenging