A Charged Particle Moving Perpendicular to a Magnetic Field Has

Multiple Choice

A charged particle moving perpendicular to a magnetic field has a magnet force on it.But when an observer moves with the particle,so that the particle has a relative velocity of zero with respect to the observer,one would expect there would be no magnetic force.Yet there is still a force.If it is not a magnetic force,then what is the source of this force?

A) It is an electrical force.
B) It is due to the ether.
C) It is a gravitational force.
D) It is something other than any of the above.