What other material could replace copper in an electric motor?
You CAN use any conductive metal instead of copper.  The question is what would happen if you did.  If the resistivity of the metal you use is less than copper (like silver) then you could 1) use smaller wire and therefore have a smaller motor to get the same performance as copper or 2) use the same size wire and get a more efficient motor.

Similarly, if you used a metal with higher resistivity than copper (like aluminum), then you would need bigger wire to maintain the same performance.  This would lead to either bigger motors for the same performance or less efficient motors. 

Aluminum has been used in for motors but it is quite rare.