The stability of the compounds 

 

Cyclohexane (iv) is non-planar and has chair conformation. In this conformation, the bond angle is the normal tetrahedral angle (109°, 28^') and thus has no angle strain and hence is most stable. The rest of the molecules are nearly planar and hence their stability depends upon the angle strain in accordance with Baeyer’s strain theory. Since cyclopropane has higher 
$$\begin{gathered} \text{ angle strain }\left(\frac{{109}^{\circ }{28}^{\mathrm{\prime }}-{60}^{\circ }}{2}={24}^{\circ },{44}^{\mathrm{\prime }}\right)\text{ than } \\ \text{ cyclopentane }\left(\frac{{109}^{\circ }{28}^{\mathrm{\prime }}-{108}^{\circ }}{2}=0^{\circ },{44}^{\mathrm{\prime }}\right)\text{. Therefore } \end{gathered}$$

cyclopentane (iii) is more stable than cyclopropane (i). Further, because of the presence of a double bond in a three membered ring, cyclopropene (ii) is the least stable. Thus the order of stability is (iv) > (iii) > (i) >(ii).