The correct answer is A. magnetic screening.
Magnetic screening is a technique used to reduce the effect of stray magnetic fields on sensitive electronic equipment. It is achieved by placing a ferromagnetic material between the source of the magnetic field and the sensitive equipment. The ferromagnetic material will absorb the magnetic field, reducing its strength at the location of the sensitive equipment.
Wagner earthing device is a device used to reduce the effects of electromagnetic interference (EMI) on electrical equipment. It works by providing a low-impedance path for EMI currents to flow, which helps to reduce the voltage drop across the equipment and the resulting interference.
Wave filters are devices used to filter out unwanted frequencies from a signal. They are often used in audio and video equipment to remove noise and interference.
Of the above options, only magnetic screening is specifically designed to reduce the effect of stray magnetic fields. Therefore, it is the correct answer to the question.