A current through a horizontal power line flows in east to west direction. What will be the direction of magnetic field at a point directly below it when viewed from east end ?

The correct answer is A, Clockwise in a plane perpendicular to the wire. Seen from the east end, the current flows away from the observer, and by the right-hand thumb rule the field lines around it are clockwise circles in a plane perpendicular to the wire.

A straight current-carrying wire produces magnetic field lines that are concentric circles around it, lying in planes at right angles to the wire. Grip the wire with the right hand, thumb pointing along the current: the curled fingers show the direction of the field. Here the current runs from east to west, so an observer at the east end sees it moving away, and the field circles appear clockwise. At the point directly below the wire, this field points towards the south; directly above the wire it points north.

B is wrong: the field would look anti-clockwise only if the current came towards the observer, that is, flowed from west to east. C is wrong: field lines around a straight wire never lie in a plane parallel to it. D is wrong for the same reason, and its sense of rotation is also wrong.

Exam tip: right-hand thumb rule: current away from you gives clockwise field, current towards you gives anti-clockwise field.