b
\((1)\) \(C{H_3} - \mathop {CH}\limits^ \bullet  C{H_3}\)

\((2)\) (fig.)

\((3)\) \(\mathop {C{H_2}}\limits^ \bullet   - CH{(C{H_3})_2}\)

\((4)\) \(\mathop {C{H_2}}\limits^ \bullet   - C{H_3}\)

The stability of free radicals is mostly affected by the factors like resonance, hyperconjugation and inductive effect. In compound \( (II)\), there is high resonance effect due to which it is highly stable than other three compounds. The effect of hyperconjugation is directly proportional to the number alpha hydrogen. So, compound \(I\) has six alpha hydrogen atoms, compound \(III \) has one alpha hydrogen atom and compound \(IV\)  has three alpha hydrogen atoms. Therefore, the order is \(II > I > IV > III.\)