Magnetic field lines form closed loops from pole to pole. This means that the lines of magnetic force always form a loop, starting at the north pole of a magnet and ending at the south pole. The direction of the magnetic field is always perpendicular to the direction of the motion of the charged particle.

Explanation:
Magnetic field lines form closed loops from pole to pole because magnetic field is a vector quantity and has both direction and magnitude. The direction of the magnetic field lines is always from the north pole to the south pole of a magnet. The magnetic field lines are usually represented by arrows that indicate the direction of the magnetic force. The magnetic field is strongest at the poles of a magnet and weakest at the equator.

The reason why magnetic field lines form closed loops is due to the fact that a magnetic field is always produced by a magnet or a moving charged particle. When a magnet is placed in a magnetic field, it produces a magnetic force that flows in a circular path around the magnet. This magnetic force is what creates the magnetic field lines.

The magnetic field lines always form closed loops because the magnetic force is a conservative force, meaning that it always follows a closed path. This means that the magnetic field lines always form a complete loop from the north pole to the south pole of a magnet.

Conclusion:
In conclusion, magnetic field lines form closed loops from pole to pole because the magnetic force is a conservative force that always follows a closed path. This closed path creates a complete loop from the north pole to the south pole of a magnet.