Explanation: A power transformer's primary winding acts as an inductive load on the AC line, causing the current to lag the voltage. This results in a poor power factor (less than 1), meaning the AC source must supply more reactive power than the actual power consumed. Placing a capacitor in series with the primary introduces capacitive reactance, which opposes and cancels some of the transformer's inductive reactance at the line frequency. By bringing the primary current and voltage closer into phase, the overall impedance presented to the AC line becomes more resistive. This reduces the reactive current drawn and significantly improves the power factor, making the load more efficient from the utility's perspective. Option B is incorrect; a series capacitor does not directly improve a transformer's output voltage regulation. Option C is incorrect; rectification (converting AC to DC) is performed by diodes, not capacitors.