b
As the molecular mass indicates it should be pentane and neopentane can only form one mono substituted alkyl halide as all the hydrogens are equivalent in neopentane 

\(\underset{Neopen\tan e}{\mathop{\begin{matrix}
   \,\,\,\,C{{H}_{3}}  \\
   |  \\
   C{{H}_{3}}-C-C{{H}_{3}}  \\
   |  \\
   \,\,\,C{{H}_{3}}  \\
\end{matrix}}}\,\xrightarrow{Chlorination}\underset{\begin{smallmatrix} 
 Only\,one\,mono-chloro \\ 
 derivative\,possible 
\end{smallmatrix}}{\mathop{\begin{matrix}
   C{{H}_{3\,\,}}  \\
   |\,\,\,\,\,\,  \\
   C{{H}_{3}}-C-C{{H}_{2}}Cl  \\
   |\,\,\,\,\,  \\
   \,\,C{{H}_{3}}  \\
\end{matrix}}}\,\)