Which planet has the strongest magnetic field, and hence the largest magnetosphere? what is its source?

Jupiter being the largest planet of the solar system, possesses the strongest magnetic field. Due to its extensively bigger size and stronger magnetic field, the magnetosphere it produces is the largest of all.

The magnetosphere is the region around the atmosphere that protects a planet from the harmful incoming solar radiations. Due to its bigger magnetosphere and huge size, Jupiter can divert the incoming radiations long before entering its atmosphere.

Due to presence of the metallic hydrogen in Jupiter, the magnetic field is so strong and it acts like a conductor conducting electricity, which is its main source.