Final answer:
Real power is transferred over a transmission line when there is current flow and a voltage drop across the line, most efficiently when the power factor is unity. High voltage transmission is used to reduce power loss in the lines.
Step-by-step explanation:
Real power is transferred over a transmission line primarily when there is current flow through the transmission line and there is a voltage drop across it. However, the most efficient transfer of power occurs when the power factor of the load is unity (option C). Real power (P) is the product of voltage (V), current (I), and the cosine of the phase angle between them, which is the power factor (cosφ). When the power factor is 1, this means the voltage and current are in phase, and thus all the power transmitted is used for real work, such as turning motors or lighting lamps. The presence of a reactive power component in the load (option D) does not directly imply that real power is being transferred since reactive power represents the power that oscillates back and forth between the source and the load, contributing to power losses but not to the actual work done.
For long-distance transmission, power is sent at high voltage to reduce power loss due to I²R losses, where R is the resistance of the transmission lines. High voltage decreases the current required to transmit a certain amount of power, hence reducing the overall power lost as heat in the lines. In practice, the generated voltages are stepped up by transformers to high values, such as 330,000 volts or more, for efficient transmission, and then stepped down at the destination for safe residential and commercial use.