Which of the following free radicals is the least stable ?

Among the given free radicals, ${\stackrel{•}{CH}}_3$ is the least stable since it has No-Hyper conjugation. Alkyl groups are examples for good electron donar it. With increasing the electronegativity, +I effect or electron releasing tendency decreases. +I effect or electron releasing tendency of some group are like methyl, ethyl, propyl.,etc. More the methyl groups more the electron density on the radical.