Question

When a charged particle enters a region of uniform magnetic field that is directed perpendicularly to the particle's velocity, the charged particle...

follows a curved trajectory of constant radius of curvature.

continues in straight-line motion.  

follows a curved trajectory that is in the shape of a parabola.

Answer 1

The correct choice is

follows a curved trajectory of constant radius of curvature.

As the charged particle enters the region of uniform magnetic field directed perpendicular to the direction of motion, it experiences magnetic force in perpendicular direction to its direction of motion as well as to the direction of magnetic field.

Since the magnetic force on the charge particle is perpendicular, only the direction of velocity of particle change and not magnitude. hence the charged particle moves in a curved trajectory of constant radius of curvature.