Which of the following free radical is most stable ?
 

• The stability of free radicals is same as that of carbocations. Like carbocations, tertiary radical is more stable than secondary and primary radical. 
• This can be explained by +I effect of the alkyl groups attached to carbon radicals. 
• The more is the number of alkyl groups attached to the carbon radical, the more is the availability of e−and more is the stabilization.

So tertiary free radical is more stabilized than secondary and primary.