b
\(CH _2= CH - CH = CH _2+ HBr \longrightarrow \underset{1,2 -addition \,product}{\begin{matrix} C{{H}_{3}}-CH-CH=C{{H}_{2}}+  \\ |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\ Br\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\ \end{matrix}} \underset{1,4-addition\, product}{\begin{matrix} C{{H}_{3}}-CH=CH-C{{H}_{2}}  \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|  \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Br  \\ \end{matrix}}\)

Under mild conditions (temprature \(=-80^{\circ} C\) ), kinetic product is the \(1,2-\) addition product and under vigorous conditions, (temp. \(=40^{\circ} C\) ) thermodynamic product is the \(1,4\) -addition product.

Hence, \(1\)-bromo -\(2\)-butene is the major product under a given condition.