Decreasing order of stability for the following radicals is :
(a) II > III > I > IV (b) III > II > I > IV
(c) III < II < I < IV (d) I < IV < II < III
Both radicals and carbocations are electron deficient because they lack an octet around the carbon atom. Like carbocations, radicals are stabilized by the electron-donating effect of alkyl groups, making more highly substituted radicals more stable and delocalization of free electron due to resonance.
Least stable free radical is vinyl and phenyl radical as there is no resonance stabilization of the electron deficiency odd electron containing p orbital.
Structure I --Vinylic free radical (least stable)
Structure II --primary allylic radical
Structure III -- tertiary allylic radical (most substituted and most stable)
Structure IV -- no resonance as odd electron is present on carbon next to sp3 carbon