b
(b)\(\begin{array}{*{20}{c}}
  {C{H_2} - OH} \\ 
  {\mathop {\mathop C\limits^| }\limits_| {\mkern 1mu} H - OH{\mkern 1mu} } \\ 
  {C{H_2}OH\,\,\,\,\,\,{\mkern 1mu} {\mkern 1mu} } 
\end{array}\mathop {\xrightarrow{{{P_4} + {I_2}}}}\limits_{{\text{Small}}{\kern 1pt} {\kern 1pt} {\text{amount}}} {\mkern 1mu} \mathop {\begin{array}{*{20}{c}}
  {C{H_2} - I} \\ 
  {\mathop {\mathop C\limits^| }\limits_| {\mkern 1mu} H - I{\mkern 1mu} } \\ 
  {C{H_2} - I} 
\end{array}}\limits_{{\text{Unstable}}} {\mkern 1mu} \xrightarrow[{ - {I_2}}]{{}}{\mkern 1mu} \mathop {\begin{array}{*{20}{l}}
  {C{H_2}{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} } \\ 
  {\mathop {\mathop C\limits^{||} }\limits_| {\mkern 1mu} H{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} } \\ 
  {C{H_2} - I} 
\end{array}}\limits_{{\text{Allyl  iodide}}} {\mkern 1mu} \)