b
\(\begin{array}{l}
Let\,a\,and\,a'\,be\,the\,acceleration\,in\,both\\
cases\,respectively.\,Then\,for\,fig\,\left( a \right),\\
T - mg = ma\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( i \right)\\
and\,2mg - T = 2ma\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( {ii} \right)\\
Adding\,\left( i \right)\,and\,\left( {ii} \right),\,we\,get\\
mg = 3ma\\
\therefore \,a = \frac{g}{3}
\end{array}\)

\(\begin{array}{l}
For\,fig\,\left( b \right),\\
\,\,\,\,\,T' - mg = ma'\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( {iii} \right)\\
and\,2mg - T' = 0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( {iv} \right)\\
\,\,\,\,\,\,\,\,\,\,Solving\,\left( {iii} \right)and\left( {iv} \right)\\
\,\,\,\,\,\,\,\,\,\,\,a' = g\\
\,\therefore \,\,\,\,\,\,\,a = \frac{g}{3}\,and\,a' = g
\end{array}\)