We use Fleming’s right-hand rule in which of these cases?

The correct option is B

While working with the electrical generator

Fleming's right-hand rule is used in determining the direction of the induced emf (and hence the direction of the induced current) when we know the direction of the magnetic field and direction of conductor's motion. In an electrical generator, mechanical energy is converted into electrical energy. In a generator, current is induced and we get electricity. Fleming's right-hand rule is used there to get the direction of the current.