Which device uses a rotating electric power source to produce a magnetic field? A. a motor
B. A step-up transformer
C. A step-down transformer
D. A generator

A. a motor

Explanation:

A motor is an electrical device or machine that converts electrical energy into mechanical energy. That is, a motor takes electrical energy to produce motion. There are different types of motors such as DC motor, induction motor and so on.

When electrical current flows through a coil, a magnetic flux is been produced, therefore when a rotating power source is supplied to coil of a motor, a magnetic field is produced which causes torque.