A regenerator does not touch the compressor or the turbine at all, it simply sits between them and the combustion chamber, catching heat from the hot exhaust gas leaving the turbine and handing it to the cooler compressed air before that air reaches the burner. Since the compressor still works between the same two pressures and the turbine still expands the gas through the same pressure drop, the compressor work input and the turbine work output are unchanged, so the net work output of the cycle stays exactly the same as before. What does change is the fuel burnt in the combustion chamber, because the air entering the burner is already partly hot thanks to the regenerator, so less heat needs to be added to reach the same turbine inlet temperature. Thermal efficiency is net work divided by heat supplied, and with the numerator fixed and the denominator (heat added in the chamber) reduced, the efficiency goes up. So the efficiency gain comes from the decrease in heat that must be supplied in the combustion chamber, option (D).