Assertion (A) : When a vector is rotated, its magnitude remains constant

  Reason (R) : The magnitude of a vector is independent of the coordinate system

Solution:

B, C, D are not correct because A vector has both directions & magnitude,

when a vector is rotated its magnitude remains constant and independent

of coordinate system.

The direction of vector is expressed in terms of an angle without any reference then the angle

is considered with positive x – axis in anticlock wise direction.