For protecting a sensitive equipment from the external magnetic field, it should be

Explanation

The correct answer is to place the equipment inside an iron case. Iron is a ferromagnetic material, which means it can absorb and redirect magnetic field lines, thus providing effective magnetic shielding. This property makes it suitable for protecting sensitive equipment from external magnetic fields. Option A, using an aluminium case, is incorrect because aluminium is not effective for magnetic shielding due to its non-ferromagnetic nature. Option C suggests using insulation, which is incorrect because insulation is used for preventing electrical conduction, not for blocking magnetic fields. Option D, using a copper sheet, is also incorrect because while copper is conductive and can provide electromagnetic interference shielding, it is not effective for shielding against static or low-frequency magnetic fields.