$(a)$ $\begin{array}{*{20}{c}}
  {\,\,\,\,\,\begin{array}{*{20}{c}}
  {\,I} \\ 
  | 
\end{array}} \\ 
  {C{H_3} - C - H} \\ 
  {\,\,\,\,|} \\ 
  {\,\,\,\,\,\,\,\,\,\,C{H_3}} 
\end{array}$ , Isopropyl iodide $(2^o)$

$(b)$ $\begin{array}{*{20}{c}}
  {\begin{array}{*{20}{c}}
  {\,\,\,\,H\,\,\,\,\,\,H\,\,\,\,\,H} \\ 
  {\,\,\,|\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,\,|} 
\end{array}} \\ 
  {H - C - C - C - I} \\ 
  {\,\,\,\,|\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,|\,} \\ 
  {\,\,\,\,\,\,\,\,\,\,H\,\,\,\,\,\,H\,\,\,\,\,C{H_3}} 
\end{array}$, sec- Butyl iodide $(2^o)$

$(c)$ $\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,\,\,\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,C{H_3}} \\ 
  | 
\end{array}\,} \\ 
  {{H_3}C - C - I} \\ 
  {\,\,\,\,\,|} \\ 
  {\,\,\,\,\,\,\,\,\,\,C{H_3}} 
\end{array}$ , tert - Butyl iodide $(3^o)$

$(d)$ $\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\begin{array}{*{20}{c}}
  {\,\,\,C{H_3}\,\,H\,\,} \\ 
  {|\,\,\,\,\,\,\,\,\,|} 
\end{array}} \\ 
  {C{H_3} - C{H_2} - C - C - Cl} \\ 
  {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,\,|\,} \\ 
  {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,CH{\,_3}\,H\,\,\,\,\,\,} 
\end{array}$, Neohexyl chloride $(1^o)$

Thus, $(d)$ is a primary $(1^o)$ halide.