Which device uses a rotating magnetic field to produce an electric current? a. a transformer b. an electromagnet c. a motor d. a generator

The correct answer is C) a motor.
In particular, we are talking about an AC motor, which produces an alternating current. In an AC motor, a coil is immersed in a rotating magnetic field. Due to the motion of the magnetic field,the angle between the direction of the field and the surface enclosed by the coil changes. As a result, the magnetic flux through the coil changes over time (the magnetic flux is given by:

where B is the intensity of the magnetic field, A is the area enclosed by the coil and  is the angle between the direction of B and the perpendicular to the plane of the coil). For Faraday-Newmann-Lenz law, this change in flux induces an electromotive force (emf) into the coil, according to:

 where the numerator is the variation of magnetic flux and dt is the time interval. This emf in the coil produced an electrical current in the circuit.

The answer is D. A generator. 

A generator is a device that converts mechanical energy into electrical energy. The mechanical energy forces the electric charges to move through the rotating wire. 

The generator follows the concept of Faraday's law. When you rotate a conductor of electricity in a magnetic field, an electromagnetic field is induced.  This creates a flow of charge. 

The answer again is D. Generator.