Pumped storage contributes to grid reliability primarily by providing:

• A. Fuel supply for base-load plants.
• B. Waste heat recovery.
• C. Long-distance transmission capacity.
• D. Short-term frequency regulation and bulk energy storage during peak demand.

Answer: (D)

Pumped storage helps grid reliability by acting as a fast-response energy storage that can quickly inject or absorb power to keep the grid frequency stable and to cover short-term imbalances. When demand spikes or a generator suddenly trips, pumped storage plants can switch to generating almost immediately, providing the rapid, flexible support needed to keep frequency within its target range. They also store energy during low-demand periods and release it during peaks, smoothing fluctuations and enhancing resilience during high-demand times.

This is not about supplying fuel for baseload plants, recovering waste heat, or expanding transmission capacity. Those functions serve different purposes, whereas pumped storage’s main benefit is rapid energy delivery and frequency regulation to maintain reliable grid operation.