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.