The stability of given free radicals in decreasing order is

1)${\mathrm{CH}}_3-\stackrel{\bullet }{\mathrm{C}}{\mathrm{H}}_2$

2)${\mathrm{CH}}_3-\stackrel{\bullet }{\mathrm{C}}\mathrm{H}-{\mathrm{CH}}_3$

3)

4)$\stackrel{\bullet }{\mathrm{C}}{H}_3$

For a given free radicals, if the alkyl groups attached are increases then positive inductive effect (+I.E) is increased and then stability also increased. Among given all free radicals, tertiary butyl radical is most stable. The decreasing order of stability of free radicals is $3>2>1>4$