Answer:
A magnet has a field having lines of force around it that moves through the air which is a nonpermeable medium that allows the passage of magnetic field lines of force. The cardboard is a similar nonpermeable medium and the magnetic lines of force pass through to induce magnetism in the iron nail which is a ferromagnetic material that becomes magnetized by the magnetic field from the magnet
The induced magnetism in the iron nails causes the reverse poles to form in the iron with the iron domain's induced north pole pointing in the direction of the south pole of the magnet and vice versa
The iron nail is then attracted to the magnet as like poles attract, thereby holding the iron nail in place with the cardboard in between, without direct contact between the nail and the magnet as the cardboard prevents the magnet from touching the iron nail held in place
Step-by-step explanation: