Butyne is the alkyne with four carbon atoms, and, as the question states, it has one carbon-carbon triple bond. The general formula of an alkyne is CnH2n-2, that means that when there are 2 carbons the formula is C2H2, when there are 3 carbons the formula is C3H4, and when there are 4 carbons the formula is C4H6. So, butyne is C4H6 or " CH3-C (triple bond) C - CH3" (note that I wrote the phrase triple bond between the two Cs in the middle of the formula because there is no way to draw the three hyphens stacked one over other").
