Conductors develop electric currents in them

The correct option is: on applying electric field.

Let us understand the behavior of conductors and how electric currents are established within them:

1. Nature of Conductors:
Conductors, such as metals, contain a large number of free electrons (charge carriers) that are not bound to any specific atom. These free electrons are in constant, random thermal motion. However, in the absence of an external force, their net movement in any particular direction is zero, resulting in no net electric current.

2. Effect of an Electric Field:
When an external electric field ($E$) is applied across a conductor, it exerts an electrostatic force ($F$) on each free electron, given by:
$F=-eE$
where $-e$ represents the charge of an electron. This force causes the free electrons to drift in a direction opposite to the applied electric field. The ordered motion of these charge carriers constitutes a net flow of charge, which is defined as an electric current.

3. Why other options are incorrect:
- Placing a conductor statically in a magnetic field does not exert a force on stationary charges, nor does it create a steady current unless there is a changing magnetic flux (which is not specified here).
- Gravitational fields exert force on mass, not on charge in a way that generates electrical current.
- Combining gravitational and static magnetic fields also fails to produce the necessary force to drive free electrons into a continuous drift current.