d
Since initially the centre of mass of the system was at rest therefore the velocities of masses \(m\) and \(2 m\) should be \(v\) and \(v / 2\). Therefore they cannot have the same kinetic energy.

Total initial energy \(=\) Total final energy When spring at its normal length then energy stored in spring becomes zero. It means total spring energy is transferred to masses kinetic energy.

\(60=\frac{1}{2} mv ^2+\frac{1}{2} \times 2 m \left(\frac{ v }{2}\right)^2\)

\(60=\frac{3}{2}\left(\frac{1}{2} mv ^2\right)\)

\(\frac{1}{2} mv ^2=40\,J\)