Which circuit electronically amplifies return signals from the receiving transducer?

The correct choice highlights the function of a receiver-amplifier circuit, which is specifically designed to enhance the signals received from a transducer. In ultrasonic testing, after the initial ultrasonic pulse is sent into the material, echoes return from various internal features such as flaws or boundaries. The transducer captures these returning signals, which are typically weak due to various factors like attenuation and distance traveled.

The receiver-amplifier circuit plays a crucial role in boosting these faint signals to a level that can be accurately analyzed, allowing for proper evaluation of the material's integrity. This amplification is vital for ensuring that subtle defects can be detected, as the quality of the received signals directly influences the reliability of the inspection results.

Other circuits mentioned serve different purposes and do not play a role in amplifying the return signals. For instance, clock circuits manage timing functions, signal generator circuits are responsible for producing the initial ultrasonic pulse, and pulse-generator circuits are designed to create and manage the transmission pulses sent to the transducer. Thus, while they are important in the overall ultrasonic testing process, they do not amplify the returning signals as the receiver-amplifier does.