The correct answer is: D. spinning reserve
Spinning reserve is the generating capacity connected to the bus bars and ready to take load when switched on. It is a measure of the ability of the power system to withstand sudden changes in demand or loss of generation.
Firm power is the generating capacity that is available to meet load during normal operating conditions. It is not available to meet sudden changes in demand or loss of generation.
Cold reserve is generating capacity that is not currently in service but can be brought on line within a short period of time. It is used to meet sudden changes in demand or loss of generation.
Hot reserve is generating capacity that is currently in service but is not providing power to the grid. It is used to meet sudden changes in demand or loss of generation.
Spinning reserve is the most important type of reserve because it can be used to meet sudden changes in demand or loss of generation. Firm power and cold reserve are not available to meet sudden changes in demand or loss of generation. Hot reserve is available to meet sudden changes in demand or loss of generation, but it is not as important as spinning reserve because it takes longer to bring on line.