Question

A charged particle would continue to move with a constant velocity in a region wherein,

Answer 1

A charged particle would continue to move with a constant velocity in a region where is no other charge is acting on it.

Answer 2

$\huge\mathbb\red{HEY \:THERE !}$

A changed particle will move with a constant velocity in a region if the force on it due to both the electric field and magnetic field is zero, or force on the charged particle due to electric field is equal and opposite to the force. on it due to magnetic field.