The correct answer is C: The rate at which RF energy is absorbed by the body. SAR (Specific Absorption Rate) measures the rate at which RF energy is absorbed by the body. SAR is expressed in watts per kilogram (W/kg) and is a key parameter for RF exposure evaluation. SAR quantifies how much RF power is absorbed per unit of body mass. It's the standard measure used to evaluate RF exposure and set MPE limits. Higher SAR means more RF energy is being absorbed, which increases the risk of heating and potential health effects. SAR depends on frequency, power density, and how the body interacts with the RF field. MPE limits are based on SAR values that are considered safe. SAR is measured in controlled laboratory conditions and is used to establish the exposure limits that amateurs must comply with.