A constant current flow in a horizontal wire in the plane of the paper from east to west as shown in the given figure. The direction of magnetic field at a point will be north to south
A) directly above the wire
B) directly below the wire
C) at a point in plane of paper, on north side of wire
D) at a point in the plane of paper, on the south side of wire.

Hint: Right hand thumb rule is used to find out the direction of magnetic field when direction of current is given. In the above case, the direction of magnetic field is perpendicular and is in the direction the fingers curl when wrapped along the current direction.

Complete answer:
The direction of magnetic field from north to south direction is at a point located directly below the wire. The direction of the magnetic field is perpendicular to the wire and is in the direction the fingers of your right hand would curl if you wrapped them around the wire with your thumb in the direction of the current.
Hence, the direction of magnetic field at a point in north to south will be directly below the wire.

Hence the correct Option is b.

Additional information:
In the right-hand thumb rule, we can find out the direction of magnetic field in a current carrying conductor. If the thumb represents the direction of current travelling in the circular wire or current carrying conductor, the curl of fingers is used to represent the direction of the magnetic field. If we grasp the conductor in the palm of their right hand so that the thumb in the given direction of flow of current, then the direction in which fingers curl, gives the direction of magnetic field lines.

Note:
In the right-hand thumb rule, the thumb points to the direction of current and the curl of our fingers show us the direction of the magnetic field. Many of us will mis concept that the thumb is the direction of the magnetic field. It is false because the magnetic field lines are always closed lines and are circular, thus, only the curl of our fingers will represent the magnetic field lines.