Acetylene and hydrogen cyanide is passed through a red hot tube 2:1 to form?

Acetylene on passing through red-hot iron tube at 873K undergoes cyclic polymerization. it trimerises to give you Benzene (C6H6). Three of the acetylene molecules polymerise to form benzene.

3C2H2 → C6H6

This is the famous Diels Alder reaction.