Step 1: Understanding the Concept:
This question asks about the fundamental cause of electric current in a conductor. An electric current is defined as the net flow of electric charge. Conductors, like metals, have a large number of free electrons that can move throughout the material.
Step 2: Detailed Explanation:
(A) on applying electric field:
When an electric field ( ) is applied across a conductor, it exerts an electrostatic force ( ) on the free charge carriers (electrons). For electrons, , so the force is . This force causes the electrons to accelerate and drift in a direction opposite to the electric field, resulting in a net flow of charge, which is an electric current. This is the primary mechanism for generating a steady current in a conductor.
(B) on placing the conductor in the magnetic field:
A magnetic field exerts a force ( ) only on charges that are already in motion ( ). A static magnetic field applied to a stationary conductor will not initiate the movement of charges, and therefore will not create a current by itself. (Note: A changing magnetic field can induce an electric field and thus a current, but the option refers to placing the conductor in a magnetic field, implying a static field).
(C) on applying gravitational field only:
A gravitational field exerts a force on mass, not on electric charge. It does not cause the ordered motion of charge carriers needed to constitute an electric current.
(D) on applying the magnetic field and gravitational field:
As explained above, neither a static magnetic field nor a gravitational field can independently initiate a current in a stationary conductor. Their combination would also not result in an electric current.
Step 3: Final Answer:
An electric current is produced in a conductor when an electric field is applied, as it provides the necessary force to cause a directed motion of free charges.