Which of the following terms describes the period required for restoring contingency reserves?

The correct choice describes the period typically required to restore contingency reserves, which is commonly understood to be ninety minutes. This timeframe is significant within system operations because it allows for adequate time to address and compensate for unexpected deficiencies or failures in generation or transmission capacity.

In the context of NERC standards, contingency reserves are meant to be quickly available to handle sudden disturbances. The reason the ninety-minute window is standard is that it aligns with the operational protocols necessary to evaluate system conditions and deploy resources effectively. Operators need enough time to assess the situation, implement the required actions to restore reserve levels, and ensure system reliability during contingencies.

The other periods listed—one hour, two hours, and fifteen minutes—do not accurately reflect the regulations and standards associated with the restoration of contingency reserves within typical system operations. For instance, fifteen minutes may refer to the immediate response time for certain ancillary services, but it does not apply to the broader timeframe necessary for full restoration of reserves. Therefore, ninety minutes stands out as the correct response for the period required for restoring contingency reserves.