b
વેગમાન સંરક્ષણ નિયમ અનુસાર,

\(  {\text{0}}\,\, = \,\,{\text{2p}}\,\,{{\hat i}}\,\, + \,\,{\text{p}}\,\,{{\hat j}}\,\,\, + \,\,\,\mathop {{{\text{p}}_{\text{3}}}}\limits^ \to  \,\,\,\,\,\)

\(\therefore \,\,\mathop {{p_3}}\limits^ \to  \,\, = \,\, - 2p\,\,\hat i\,\, - \,\,p\,\,\hat j \)

\(  \therefore \,\,\left| {\mathop {{p_3}}\limits^ \to  } \right|\,\, = \,\,p\,\sqrt {{{( - 2)}^2}\, + \,\,{{( - 1)}^2}} \,\, = \,\,p\,\sqrt {4\,\, + \,\,1} \,\,\,\,\,\,\,\)

\(\therefore \,\,\left| {\mathop {{p_3}}\limits^ \to  } \right|\,\, = \,\,p\,\sqrt 5 \)