The statement that correctly describes magnetic field lines is:
OB. They form loops.
Magnetic field lines are continuous loops that represent the direction and strength of the magnetic field. They are typically depicted as lines that form closed loops, extending from the north pole of a magnet to the south pole. These loops help visualize the direction in which a compass or a magnetic object would align itself in the presence of a magnetic field.
Option OA, stating that magnetic field lines are parallel with one another, is incorrect. Field lines do not run parallel to each other but form continuous loops.
Option OC, stating that magnetic field lines join north pole to north pole, is also incorrect. Magnetic field lines connect the north pole of a magnet to the south pole, forming closed loops.
Option OD, stating that magnetic field lines show the direction a south pole will point, is also incorrect. Magnetic field lines indicate the direction in which a compass needle or a south pole of a magnet would align, not the direction a south pole will point.