Correct Answer  Option 1 : 2 MJ / MVA
Two machine system:
 In a twomachine system, a synchronous generator is connected to a synchronous motor by a lossless network.
 Initially, both the machines are in stable condition. When the load on the motor increases suddenly, the electrical input cannot change simultaneously so that the motor will get oscillations.
 When the oscillations are very high when compared to the natural frequency of oscillations the motor has become unstable.
 When the motor has become unstable, the load on the synchronous generator is zero and the generator will make oscillations.

Then the above two machine system becomes a single machine system.
If the two machines are swinging together then the equivalent inertia constant is given by
Heq = H1 + H2
Explanation:
As given both the machines having the same inertia constant
H1 = H2 = H
Effective inertia constant = 4 MJ/MVA
Heq = 4 MJ/MVA
Heq = H + H
Heq = 2H
4 = 2H
H = 2 MJ/MVA