The correct answer is: B. rated head
The rated head is the net head under which the turbine reaches its peak efficiency at synchronous speed. It is the head that the turbine is designed to operate at and is typically specified by the manufacturer.
The design head is the head that the turbine is designed to operate at under ideal conditions. It is typically higher than the rated head to account for variations in the head due to changes in the water flow and the turbine’s operating speed.
The gross head is the total head that the turbine experiences. It is the difference between the upstream and downstream water levels.
The operating head is the head that the turbine is actually operating at. It can be different from the rated head due to changes in the water flow and the turbine’s operating speed.