A(n) _____ is an instrument that graphically displays an instantaneous voltage trace.

An oscilloscope is an instrument specifically designed to graphically display voltage over time, allowing users to observe the waveform of electrical signals. It provides a visual representation of the instantaneous voltage levels, which can be crucial for analyzing the behavior of electrical circuits. The display shows how the voltage changes in real time, which is essential for understanding signal characteristics such as frequency, amplitude, and any waveform irregularities.

This capability distinguishes oscilloscopes from other instruments, such as analog multimeters and digital multimeters (DMMs), which typically display voltage readings in numerical form rather than as a dynamic graphical representation. A frequency counter, on the other hand, measures the frequency of an oscillating signal but does not provide any visual waveform information. Thus, the oscilloscope is uniquely suited for applications where real-time monitoring of voltage waveforms is necessary, making it the correct answer for this question.