De or het mountainbiking?

The correct Dutch article is:

de mountainbiking

Rule Words ending in -ing: almost always de. See the rule
Meaning (Dutch) een tak van de wielersport waarbij je fietst over onverharde, ruige terreinen, zoals bossen, bergen en zandpaden.

How to use mountainbiking

the mountainbikingde mountainbiking
this mountainbikingdeze mountainbiking
that mountainbikingdie mountainbiking
our mountainbikingonze mountainbiking
your mountainbikingjouw mountainbiking
every mountainbikingelke mountainbiking
which mountainbiking?welke mountainbiking?
a big mountainbiking / the big mountainbikingeen grote mountainbiking, de grote mountainbiking

Meaning, plural, diminutive, pronunciation and translations: WikiWoordenboek (Dutch Wiktionary) (CC BY-SA 4.0). Example sentences: Tatoeba (CC BY 2.0 FR).