The diagram represents magnetic field lines within a region of space. The magnetic field is strongest at point

The magnetic field is strongest at point C.

Explanation:

In a magnetic field, the center is known to be strongest region of the magnetic field while between the two poles outside the bar magnet are the weakest region.

Lines of the magnetic field are seen to be the densest even at the center of the magnetic field while between the poles outside the bar magnet, it is least dense.

A magnetic field is known to be a field which shows the magnetic impact and influence on electric charges, magnetic materials and electric currents.