Hydrocarbons are composed of carbon and hydrogen atoms and are generally non-polar. The only intermolecular force of attraction between hydrocarbons is the weak van der waals interaction. Longer the hydrocarbon chain, stronger will be this interaction.
In the given options:
Propane = C3H8
Octane = C8H18
Hexane = C6H14
Ethane = C2H6
Thus, octane is has the longest chain and will have the strongest van der waal interaction which will allow the molecule to stay in a liquid state longer.
