Octane number indicates the anti-knock properties of fuels. Higher the octane number, more difficult the auto-ignition. Octane number of straight chain alkane are lower than that of branch chain alkanes, while aromatic hydrocarbons show higher value of octane number. The correct order of octane number is as follows: Alkane < alkenes < cycloalkanes < aromatic hydrocarbons Thus, (d) is the correct option.