Which of the following devices would be used to convert alternating current for specific motor applications?

The most relevant device for converting alternating current (AC) for specific motor applications is the combination of SCR drives (Silicon Controlled Rectifiers) and VVVF drives (Variable Voltage Variable Frequency drives).

SCR drives are used to control the power delivered to an AC motor by adjusting the voltage and switching the current on and off at specific intervals. This capability allows for precise control over the speed and torque of the motor. Similarly, VVVF drives change the frequency and voltage of the input power to manage the motor's operation more efficiently. This is crucial for applications that require varying speeds or precise control of motor dynamics.

These drives are specifically designed for motor control, making them ideal for various industrial and commercial applications, where the need to modify speed, power, or even direction is essential for efficient operation. The other devices listed, while useful in other contexts of electrical engineering, do not serve this specific function of converting or controlling AC power for motor applications.