a
\(\begin{matrix}
   \,\,\,\,C{{H}_{3}}  \\
   |  \\
   C{{H}_{3}}-C-C{{H}_{2}}  \\
   |  \\
   H  \\
\end{matrix}\)  \(\xrightarrow[B{{r}_{2}}]{hv}\) \(\underset{\begin{smallmatrix} 
 2-bromo-2-methyl\,bu\tan e \\ 
 (major\,product) 
\end{smallmatrix}}{\mathop{\begin{align}
  & \begin{matrix}
   \,\,\,\,C{{H}_{3}}  \\
   |  \\
   C{{H}_{3}}-C-C{{H}_{2}}  \\
   |  \\
   H  \\
\end{matrix}\xrightarrow[B{{r}_{2}}]{hv} \\ 
 & \begin{matrix}
   C{{H}_{3}}\,\,\,\,\,\,\,\,\,  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   C{{H}_{3}}-C-C{{H}_{2}}-C{{H}_{3}}  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   Br\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
\end{matrix} \\ 
\end{align}}}\,\)

The reactivity of different types of hydrogen folloes the order : benzylic \(\approx \) allylic \(>\) tertiary \(>\) secondary \(>\) primary \(>\) vinylic \(\approx \) aryl.