A current through a horizontal power line flows east to west direction. What is the direction of the magnetic field at a point directly below it?

Hint: We should first understand the position of the point with respect to the power line and the direction of flow of current. Then by using Maxwell’s right hand thumb rule, we can find the direction of the magnetic field at that point.

Complete step-by-step answer:
We have been given a horizontal power line in which the current is flowing from east to west and we need to find the direction of the magnetic field at a point directly below the power line.
We will first understand Maxwell's right hand thumb rule, which states that if we hold a current conducting wire with our hand such that the thumb points along the direction of current and if we curl our other four fingers then that will denote the direction of magnetic field lines. This can better be understood from the diagram below.
 

The curled arrow shows the direction of magnetic field lines and I is the current flowing through it.
Now, let us illustrate what given in the question,



Here, the power line has current I flowing from east to west and has been placed horizontally and we need to find out the direction of the magnetic field at point P which is just below it.
So, if we curl our fingers keeping the thumb in the direction of current flow, we will see that the direction of the magnetic field line at point P will direct from north towards south.
Hence, the direction of the magnetic field at a point directly below the power line will be from north towards south.

Note: If we do not understand the proper orientation of the power line and the point, the answer will be different. Like many will think as if the direction of the magnetic field will be outwards of the plane in which the power line has been kept. So, proper understanding of the positions is must.