The correct order of octane number of butane, pentane, hexane and cyclohexane is (a) butane > pentane > hexane > cyclohexane (b) butane > pentane > cyclohexane > hexane (c) butane > cyclohexane > pentane > hexane (d) cyclohexane > butane > pentane > hexane

The correct answer is Option 3: butane > cyclohexane > pentane > hexane. The octane number measures a fuel's resistance to knocking, which is heavily influenced by molecular structure.

• Chain Length: For straight-chain alkanes (n-alkanes), the octane number decreases as the carbon chain length increases. Therefore, butane (C4) has a higher octane number than pentane (C5), which is higher than hexane (C6).
• Molecular Structure: Cyclic compounds and branched isomers possess significantly higher octane numbers than their straight-chain counterparts. Cyclohexane, being a cyclic alkane, is more resistant to knocking than straight-chain pentane and hexane.
• Relative Ranking: While butane (a short-chain gas) has a very high octane rating (approx. 94), cyclohexane (approx. 83) follows. Pentane (approx. 62) and hexane (approx. 25) rank lowest due to their longer, linear chains which ignite more easily under pressure.

Thus, the decreasing order is butane > cyclohexane > pentane > hexane, making Option 3 the accurate representation of their chemical properties.