why No force is exerted by the magnetic field on the arms AD and BC.? inrectangular loop.?

electric motor works on the principle that when an electric current is passed through a conductor placed normally in a magnetic field a force acts on the conductor. The direction of the force is obtained with the help of Fleming's left hand rule.

Arm BC and AD  are parallel to the magnetic field.that is why no force will  act on BC and AD.