The question asks to identify the specific device used for the task of receiving and displaying ultrasonic signals. Ultrasonic signals are sound waves with frequencies higher than the upper audible limit of human hearing (typically above 20 kHz). To answer this, we need to understand the function of each available option in relation to signal processing.

Ultrasonic Signal Fundamentals

Ultrasonic signals, while originating as sound, are often converted into electrical signals by transducers before they can be processed or displayed by electronic devices. The device in question must be capable of handling these electrical representations of ultrasonic waves and presenting them in a visible format.

Analyzing Device Options for Signal Reception and Display

Let's examine each option:

• Optical microscope: This device uses light (visible spectrum) to magnify extremely small objects. It operates on principles of optics and is unrelated to the detection or display of sound or ultrasonic waves.
• Cathode ray oscilloscope (CRO): A CRO is an instrument primarily used to observe and analyze the characteristics of electrical signals. It plots voltage (on the vertical axis) against time (on the horizontal axis), creating a visual waveform. If an ultrasonic transducer converts ultrasonic signals into electrical signals, a CRO can effectively display this waveform, allowing for analysis. Therefore, it fits the criteria of receiving (the electrical signal) and displaying the characteristics of the original ultrasonic event.
• Photodiode sensor: This is a semiconductor device that converts light into an electrical current or voltage. Its function is related to light detection, not sound or ultrasonic waves.
• Electromagnetic coil: While coils are fundamental components in many electronic devices (like speakers, transformers, inductors), they are not inherently designed as a standalone system to specifically receive and display ultrasonic signals in a visual format. They might be part of a larger system (like a transducer), but not the complete solution for display.

Conclusion on Displaying Ultrasonic Signals

Based on the analysis, the Cathode Ray Oscilloscope is the most suitable device among the choices for visually displaying electrical signals that represent ultrasonic signals after they have been converted by a suitable transducer.