Fleming's left hand rule is used to determine what?

Fleming's left-hand rule is a useful mnemonic that helps to determine the direction of force experienced by a current-carrying conductor in a magnetic field. When you align your thumb, first finger, and second finger of your left hand perpendicular to each other, the thumb represents the direction of the force (or motion) on the conductor, the first finger indicates the direction of the magnetic field, and the second finger shows the direction of the current.

In this context, option B correctly states that the rule helps in identifying the movement of a wire in a magnetic field, which is particularly important in understanding how electric motors operate. When a current passes through a wire placed in a magnetic field, the interaction between the magnetic field and the current generates a force that moves the wire, making the application of Fleming's left-hand rule integral in such scenarios. This is why option B is the right choice.