What happens to the cabin VSI when depressurisation occurs at FL310 due to the outflow valve opening?

During a depressurization event at flight level 310, the cabin altitude begins to rise due to the opening of the outflow valve relieving the pressure inside the cabin. This process causes the pressure within the cabin to decrease until it equals the ambient pressure outside at that flight level, which is essentially zero pressure at high altitudes.

As the cabin altitude increases, the cabin vertical speed indicator (VSI) reflects this change. The cabin altitude rises until the pressure differential between the cabin and the outside environment is zero. Once this point is reached, cabin altitude effectively becomes static with respect to the external ambient pressure, which leads to the VSI indicating a descent that aligns with the rate at which the aircraft descends.

Thus, the cabin VSI, after reaching a zero pressure differential, follows the aircraft's vertical speed, indicating a descent that is equal to the aircraft VSI, since both the cabin and the external ambient pressure have become equal at that point. This is why the correct answer indicates that the cabin VSI shows a descent equal to the aircraft VSI after the cabin altitude reaches its maximum increase due to loss of pressurization.