The correct answer is D. All of above.
A CRO, or cathode ray oscilloscope, is an electronic test instrument that is used to display waveforms of electrical signals. It can be used to measure voltage, frequency, and phase of an electrical signal.
A CRO works by sweeping an electron beam across a phosphorescent screen. The beam is deflected by the electrical signal being measured, causing the trace of the signal to be displayed on the screen. The voltage of the signal is proportional to the brightness of the trace, the frequency of the signal is proportional to the speed of the trace, and the phase of the signal is proportional to the position of the trace on the screen.
CROs are used in a variety of applications, including electronics troubleshooting, signal analysis, and waveform generation.