A uniform B-field is is directed along the +z axis. A charged particle is moving in the Field. It takes on a clockwise circular path of decreasing radius in the xy plane. Which of the following must be true?

When any charged particle enters into magnetic field then external magnetic force acts on it called as magnetic force. It can be given as, 

Expression for magnetic force can be given as, 

$F = qvB .... \left(1\right)$

Since, charge is tracing a circular path then force acting on it can be given as, 

$F =\frac{ mv²}{r} .... \left(2\right)$

On comparing both equations, 

$v =\frac{ qB}{m}r$

This is expression for velocity of charge particle in magnetic field.