In the context of NERC regulations, what is required for regulating reserves that are part of spinning reserves?

The requirement for spinning reserves, according to NERC regulations, is that they must be AGC (Automatic Generation Control) responsive and online. Spinning reserves refer to the generating capacity that is online and can be quickly adjusted to respond to changes in demand or unexpected losses of generation. Being online ensures that the resources are immediately available to provide support, which is critical in maintaining the reliability of the grid.

AGC responsiveness means that these resources can automatically increase or decrease output when necessary, allowing for rapid adjustments to generation levels to maintain balance between supply and demand. This capability is essential during events such as sudden generator failures or unexpected spikes in load, helping to prevent outages or system instability.

The other options do not meet the specific criteria outlined for spinning reserves; resources that are offline or not responsive would not be able to provide immediate support, and the requirement of exceeding 500 MW is not a standard regulation, as the capacity for spinning reserves can vary based on system needs and configurations. Therefore, the focus on these reserves being both online and AGC responsive is what makes this choice the correct answer.