Magnetic lines of force determines:
(A) The shape of the magnetic field
(B) Only the direction of the magnetic field
(C) Only the relative strength of the magnetic field
(D) Both the direction and the relative strength of the magnetic field

Hint : To solve this question, we need to take the help of an example of the magnetic field lines due to a bar magnet. Then, by carefully exploiting each and every information indicated by them, we can get the final answer.

Complete step by step answer
We know that the field lines are the path traversed by a particle which is acted upon by the force applied by that field. So the magnetic field lines are basically the locus of a moving charge particle in a region of the magnetic field present.
The magnetic field lines for a par magnet are as shown below.

As we can clearly see from the field lines of the bar magnet, these are closely spaced at the points located near the poles. But for the points located far away from the poles, the magnetic field lines separate. This is due to the fact that the relative strength of the magnetic field at the points near to the poles is greater than that for the points far away from the poles.
So the magnetic field lines can be used to determine the relative strength of the magnetic field.
Now, the arrows present on each of the magnetic field lines indicate nothing but the direction of the magnetic field.
So the magnetic field lines can also be used to determine the direction of the magnetic field.
Thus, the magnetic lines of force determine both the direction and the relative strength of the magnetic field.
Hence, the correct answer is option D.

Note
As we can see from the above magnetic field lines for the bar magnet that the magnetic field lines are also indicating the shape. But the shape is defined for the field lines only and not for the field itself. Hence the option A is incorrect.