So in this example question it says classify each of the following hydrocarbons as an alkane, alkene or alkyne. So remember an alkane is when we have carbons just single bonded to one another. If we take a look at the options, remember every line connects 2 carbons to each other. In this option B all of them are single bonded to one another. So they're all alkanes.
Well, it is an alkane. This is the only alkane that we have and alkene is when we have carbons double bonded to one another. These two carbons here are double bonded to each other, and as long as two carbons are double bonded to each other, it is an alkene. These two carbons are double bonded to each other, so this is also an alkene.
An alkyne is when we have two carbons triple bonded to one another. We have a carbon here and a carbon here, so option C would represent an alkyne. So this is how we describe our options ABC and D from the following example question.