Explanation

(d)
In cyclopropane the angle strain is maximum. Hence, it is highly strained molecule and consequently most unstable. The angle strain in cyclobutane is less than cyclopropane. Hence, cyclobutane is more stable. This stability increases up to 6 membered rings then decreases from 7 to 11 membered rings and from the 12 membered rings onwards attains the stability of 6 membered ring. Heat of combustion is a method of measuring chemical stability. Hence, cyclohexane has the lowest heat of combustion.