The correct answer is A) thyristors. In a solid-state "soft" starter for a three-phase induction motor, as shown in the illustration, the devices that are controlled by the gate control circuits are thyristors. Thyristors are semiconductor devices that act as electronically controlled switches, allowing them to control the flow of current to the motor windings during the starting process. This "soft start" feature helps to reduce the high inrush current and torque associated with direct across-the-line motor starting, protecting both the motor and the electrical system. The other answer choices are incorrect because varactors are variable capacitance diodes, semiconductor diodes are simply rectifying devices, and zener diodes are used for voltage regulation, not for motor control applications.