Answer:
d. a large region outside Jupiter occupied by its magnetic field and filled with high-energy charged particles.
Explanation:
A magnetic field is generated by the movement of a charged particle in the space around it. For the case of Jupiter its magnetic field is created by the liquid metallic hydrogen in its core.
So the magnetosphere is just the magnetic field around a planet, which interacts with high-energy charged particles (for example: Cosmic Rays).
Magnetospheres protect planets from the extreme radiation coming from stars or another interstellar source.