Final answer:
Step-up transformers are used in the transmission of electricity over long distances to increase voltage, thereby reducing current and minimizing power loss due to resistive heating.
Therefore, the correct answer is: option (d) (a ) and (c) both.
Step-by-step explanation:
The large-scale transmission of electrical energy over long distances involves the use of step-up transformers to increase the voltage output of the generator, which is mainly done to minimize power losses during transmission. When voltage is increased (stepped-up), the current in the power lines decreases proportionally.
This reduction in current leads to a decrease in power loss due to resistive heating, which is proportional to the square of the current (according to Joule's law). Essentially, higher voltages result in lower currents, which in turn, lead to reduced power losses (Joule losses) in the transmission lines.
Thus, the correct answer to the student's question would be (d) (a ) and (c) both, meaning that the voltage output of the generator is stepped-up because of the reduction of current and the resulting decrease in power loss.