Which cycloalkane has the lowest heat of combustion per ${\mathrm{CH}}_2$ group

In cyclopropane. the angle strain is maximum hence it is a highly strained molecule and consequently most unstable. The angle strain in cyclobutane is less than that in cyclopropane. hence cyclobutane is more stable than cyclopropane. This stability increases up to 6 membered rings. then decreases from 7 to 11 membered rings and from the 12 membered 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 per ${\mathrm{CH}}_2$ group.