The angle strain is maximum in cyclopropane. Hence, it is highly strained molecule and

consequently most unstable. The cyclobutane is more stable. Hence; angle strain in cyclobutane is less than cyclopropane. This stability increase up to 6 membered rings, then decreases from 7 to 11 membered rings and from the 12 membered rings onwards attains the satbility of 6 membered rings. Heat of combustion is a method of measuring chemical stability. Hence; cyclohexane has the lowest heat of combustion.