The diagram shows the magnetic field surrounding a bar magnet. the closer together the lines of force, the stronger the magnetic field. according to the diagram, where is the magnetic field the strongest?
a. around the north pole
b. around the south pole
c. around both poles
d. in the center of the magnet

The magnetic field is strongest around both poles.

Explanation :

A bar magnet is shown in the given picture. We know that the magnetic field lines start from the north pole of the magnet and terminates at the south pole.  

The magnetic field lines are concentrated at the poles. Due to the crowdedness of the field lines, the strength of the magnetic field is greatest at the poles.

As we move away from the poles, the strength of field lines goes on decreasing.

So, the correct option is (C) " Around both poles ".