Iron cylinder induces magnetization from bar magnet

Steps:

• Bar magnet creates external magnetic field lines entering the hollow iron cylinder.
• Iron, being ferromagnetic, gets magnetized by the field, aligning internal domains.
• Magnetized cylinder produces its own field opposing the external one inside the hollow space.
• Resulting field pattern inside shows modified lines due to induced magnetization.

Why D is correct:

• Magnetic induction magnetizes ferromagnetic materials in an external field, per the definition of induced magnetism in soft irons.

Why the others are wrong:

• A: Electromagnetic induction requires changing magnetic fields, not static bar magnet.
• B: Electromagnet generation needs current in coils, absent here.
• C: Electrostatic induction applies to electric charges, not magnetic fields.

Final answer: D