What device is used to graphically display the back-and-forward resistance characteristics of a crystal diode?

The correct choice for graphically displaying the back-and-forward resistance characteristics of a crystal diode is the oscilloscope. An oscilloscope allows for the visualization of voltage over time, enabling the user to see how a diode behaves in response to an applied voltage. By connecting the diode to the oscilloscope in a test circuit, one can observe the current-voltage characteristics, including both the forward bias region, where the diode conducts, and the reverse bias region, where it does not conduct. This capability is essential for understanding the semiconductor properties of the diode and for troubleshooting circuit designs that include diodes.

While a multimeter can measure voltage and current, it does not provide a graphical representation, which limits the understanding of a diode's performance characteristics over varying conditions. A chart recorder could theoretically serve to record the behavior of a diode but does not provide the real-time visual feedback that an oscilloscope does. A frequency generator, on the other hand, is used to provide varying frequencies to a circuit but does not show the resistance characteristics of the diode itself. Thus, the oscilloscope is the most effective tool for this specific purpose.