D

Step 1: Understand the structure of a bar magnet. A bar magnet has two ends known as poles: a north pole (N) and a south pole (S).
Step 2: The magnetic field lines represent the direction and strength of the magnetic field around the magnet.
Step 3: The direction of magnetic field lines is conventionally taken to be from the north pole to the south pole outside the magnet.
Step 4: Inside the magnet, the magnetic field lines move from the south pole to the north pole.
Step 5: Therefore, the complete description of the magnetic field direction is that it goes from south to north inside the magnet and from north to south outside the magnet.
Conclusion: The correct answer is option D.