Magnetic field is stronger at poles because magnetic field lines are very close to each other near the poles. The closer the magnetic field lines are, the stronger the magnetic field at that point. The magnetic field lines are closer or denser at the poles. So, the magnetic field is strongest around the poles of the magnet. Key PointsPlacing a bar magnet in a box filled with iron fillings is aligned with this lesson objective as: A bar magnet is taken which is in the shape of a bar which remains straight without ant curves. A box is taken and some tiny iron filings are collected and put in the box. The magnet is put in the box and the students can observe that a maximum number of iron filings get attached to the poles of a magnet. This is because the magnetic field is stronger at the poles and magnetic field lines are very close to each other near the poles. Finally, students can conclude that the magnetic field is stronger at the poles . ​Hence placing a bar magnet in a box filled with iron fillings is aligned with this lesson objective.Additional Information On the other other hand remaining three activities are not suitable as they focus on the properties of a magnet but not on the point why the magnetic strength is maximum at the poles of a magnet.