Final answer:
The back of a valid California driver license contains a magnetic strip that stores the license holder's information, which is read through electromagnetic induction when swiped.
Step-by-step explanation:
The feature located on the back of a valid California driver license is a magnetic strip, also known as a magstripe. This black strip is a thin layer of magnetic material that stores the license holder's information. When a driver's license is swiped through a card reader, the information encoded on the magnetic strip is read due to a process known as electromagnetic induction.