Which statement correctly describes magnetic field lines?
a. they join north pole to south pole.
b. they always point away from a south pole.
c. they cross each other at right angles.
d. they show the direction a south pole will point.

Magnetic field lines are presentation of magnetic field on paper

There is no real existence of magnetic field lines in the surrounding of a magnetic material but it is an hypothetical way to represent the magnetic field variation.

There are several rules to draw the field lines on paper which are as following

1. magnetic field lines start from north pole and ends at south pole

2. density of field lines shows the strength of magnetic field

3. tangent to the magnetic field line represents the direction of magnetic field so there can not exist a point where two magnetic field lines will intersect

so as per all above rules the correct answer should be

A. They join north pole to south pole.

it may be wrong but i think its a

c. magnitude and direction