Explanation: Transformers operate based on the principle of electromagnetic induction, where the ratio of voltages across the primary and secondary coils is directly proportional to the ratio of the number of turns in those coils. This relationship is expressed by the formula: $V_S / V_P = N_S / N_P$ Where: * $V_S$ = Secondary voltage * $V_P$ = Primary voltage * $N_S$ = Secondary turns * $N_P$ = Primary turns Given: * $V_P = 100 \text{ V}$ * $N_P = 200$ * $N_S = 40$ Rearranging the formula to solve for $V_S$: $V_S = V_P \times (N_S / N_P)$ $V_S = 100 \text{ V} \times (40 / 200)$ $V_S = 100 \text{ V} \times (1/5)$ $V_S = 20 \text{ V}$ Since the secondary turns (40) are fewer than the primary turns (200), this is a step-down transformer, and the secondary voltage will be lower than the primary voltage. This confirms that 20 V is the correct result. The other options are incorrect because they do not follow this fundamental transformer turns ratio principle.