C

Step 1: Understand the function of each device:
- **Dynamo**: Converts mechanical energy into electrical energy.
- **Generator**: Generally refers to similar devices like dynamos that convert mechanical energy to electrical energy.
- **Electric Motor**: Converts electrical energy into mechanical energy by electromagnetism.
- **Induction Coil**: Used to step-up voltage; does not directly convert electrical to mechanical energy.

Step 2: Identify which device specifically performs the desired conversion:
Among the options, the **Electric motor** is the device that converts electrical energy into mechanical energy.

Therefore, the correct answer is C.