Which of the following free radical is/are less stable than benzyl free radical ?

Allyl radical is more stable than a,b,c,d.A free radical maybe defined as an atom or group of atoms which contains an unpaired electron and it is paramagnetic in nature. They are formed by homolysis. If the unpaired electron is on carbon atom, they are called alkyl free radicals. Free radicals are highly reactive due to their unpaired electrons. They are usually formed at high temperature or in the presence of peroxides.