Explanation: The principle governing maximum power transfer from a source to a load is known as the **Maximum Power Transfer Theorem**. This fundamental theorem states that for a given power source with a fixed internal resistance (or impedance), the maximum amount of power will be delivered to the load when the load resistance (or impedance) is equal to the internal resistance (or impedance) of the source. * **Option B is correct** because it precisely states this theorem. When the load impedance matches the source's internal impedance, the power delivered to the load is maximized. This concept is vital in amateur radio for efficient system design, such as matching a transmitter's output impedance to an antenna's impedance for optimal power radiation. * **Options A and C are incorrect** because if the load impedance is either greater than or less than the source impedance, the power transferred to the load will be less than the maximum possible. While power will still be transferred, it won't be at its peak. * **Option D is incorrect** because the internal impedance is an intrinsic characteristic of the power source itself and is critically important in determining how much power it can deliver to a connected load.