Alkynes are _____ hydrocarbons

Hydrocarbons are made up of Carbon $\left(\mathrm{C}\right)$ and Hydrogen $\left(\mathrm{H}\right)$ atoms connected by a chemical bond.

The compounds containing Carbon atoms have multiple bonds between two Carbon-carbon atoms are called unsaturated hydrocarbons

Alkynes have Carbon-Carbon atoms connected through triple and single bond, Carbon-Hydrogen atoms that are only connected through single bonds.

So, Alkynes contain a triple bond between Carbon so they are unsaturated in nature.

Final answer:

Alkynes are unsaturated hydrocarbons