Which of the following is one way that an electric motor's rotation speed can be controlled?

by varying the magnetic fields

by varying the amount of current flowing to the motor

by varying the magnetic poles

cannot be controlled

Depending on which type of motor you're talking about, but the first 3 are true.  A stronger magnetic field in a DC motor will slow it down but increase its torque.
The amount of current in the motor will control the magnetic fields and therefore affect the speed (and torque).  In an induction motor, the rotational speed is given by   where f is the line frequency and p is the number of poles.  Thus fewer poles makes it go faster.