Which compound is most reactive, ethene, ethane, or ethyne?

Ethyne is more suitable answer.

Explanation:

Among all the three, alkanes are least reactive, as they have all the saturated bonds between carbon carbon atoms. They have no polarity in them.

Alkenes have one pi bond between two carbon atoms while alkynes have two pi bonds between two carbons.

The pi bond is easy to break, so generally we consider that due to two pi bonds alkyne are more reactive than alkenes.

However, it should be noted that in electrophilic addition reaction, alkene are more reactive than alkynes.