Transformers need iron core and AC for induction

Steps:

• Recognize the device as a transformer, which steps down voltage for a low-voltage lamp.
• Note that transformers rely on electromagnetic induction via changing magnetic fields.
• Identify the core material: must be ferromagnetic like iron to concentrate magnetic flux and enable efficient coupling between coils.
• Determine power supply: AC is required to produce alternating magnetic fields for voltage induction; DC produces static fields with no induction.

Why D is correct:

• Transformers operate on Faraday's law of electromagnetic induction, requiring an iron core for high permeability and AC supply for time-varying flux to induce emf in the secondary coil.

Why the others are wrong:

• A: Plastic rod has low permeability, preventing magnetic linkage; DC supply yields no changing flux for induction.
• B: Iron core works for flux but DC supply creates constant field, inducing no voltage in secondary.
• C: Plastic rod fails to link coils magnetically, so no transformation occurs even with AC.

Final answer: D