Transformers preserve AC frequency

Steps:

• Identify transformer type: step-down due to fewer secondary turns (40 vs. 4000).
• Recall transformer principle: voltage ratio equals turns ratio, but frequency remains unchanged.
• Note input is AC at 50/60 Hz (standard supply); output current in lamp is also AC.
• Conclude output current matches input frequency, as magnetic flux alternation drives both coils equally.

Why D is correct:

• Faraday's law states induced EMF frequency equals input AC frequency, so secondary current oscillates at the same rate.

Why the others are wrong:

• A: Transformers cannot increase frequency; they only step voltage up/down.
• B: No mechanism in ideal transformers to decrease frequency.
• C: Output is fully alternating, not unidirectional like rectified DC.

Final answer: D