c
When the system is in equilibrium, then the spring force is \(3 mg\). When the string is cut, then net force on block \(A\)

\(=3 m g-2 m g=m g\)

Hence, acceleration of block \(A\) at that instant

\(a=\frac{\text { force on block } A}{\text { mass of block } A}=\frac{m g}{2 m}=\frac{g}{2}\)

When string is cut, then block \(B\) falls freely with an acceleration equal to \(g\).