Explanation: A transformer's efficiency is the ratio of its output power to its input power. Any energy not transferred to the output is lost, usually as heat. All the listed factors contribute to these losses: * **A) Eddy current losses:** These occur when circulating currents are induced in the transformer's core material itself by the changing magnetic flux. These currents generate heat, wasting energy. Laminated cores are used to minimize these losses. * **B) Hysteresis losses:** These losses are due to the energy expended in repeatedly magnetizing and demagnetizing the core material as the alternating current reverses. This constant re-alignment of magnetic domains also generates heat. * **C) Copper losses (I²R losses):** These are due to the electrical resistance of the copper wire in both the primary and secondary windings. As current flows through these windings, power is dissipated as heat (P=I²R), reducing the power available at the output. Since all these losses convert useful electrical energy into unwanted heat, they collectively reduce the transformer's overall efficiency. Therefore, all of them determine the efficiency.