(a) Fleming's left-hand rule: Stretch the forefinger, middle finger and the thumb of your left hand such that they are mutually perpendicular to each other. If the forefinger indicates the direction of magnetic field and the middle finger indicates the direction of current, then the thumb will indicate the direction of motion of the conductor or the force acting on the conductor.
(b) The displacement of the rod is largest when the direction of current is at right angles to the direction of the magnetic field.