The correct option is B three times
For an ideal gas P×V=N×K×T
where P is the pressure, V is volume of the cubical vessel,
N is no. of molecules, K Boltzmann's constant and T is temperature
So as N increases to 3N, pressure also increases by three times.
And the total energy for an ideal gas E=32×N×K×T
Hence the total energy also increases by three times.