The correct answer is A) Combustor. In a gas turbine engine, the majority of the energy is added to the working fluid (typically air) in the combustor. The combustor is where the fuel is injected and burned, significantly increasing the temperature and pressure of the working fluid before it enters the turbine section. This high-energy, high-pressure air is then used to drive the power turbine, which in turn drives the compressor and other engine components. The other options are incorrect because the compressor (C) increases the pressure of the air but does not add significant energy, the high-pressure turbine (D) extracts energy from the air rather than adding it, and the power turbine (B) uses the energy extracted from the air to produce useful work, but does not add the majority of the energy to the system.