Explanation: An oscilloscope is a crucial tool in amateur radio for visualizing electrical signals, primarily voltage, as a function of time. It can easily **measure electrical voltage** (D), as that is its primary function displayed on the vertical axis. By measuring the voltage drop across a known resistor, **electron flow (current)** (A) can be calculated using Ohm's Law (V=IR). With multiple input channels, an oscilloscope readily displays two signals simultaneously, enabling direct observation and **measurement of their phase difference** (B). However, an oscilloscope is not designed to directly **measure the velocity of light** (C). While it can measure electrical signals associated with light (like from an LED or photodiode), measuring the speed of light requires highly specialized equipment for precise time-of-flight measurements over significant physical distances, which is beyond the direct capability of a standard oscilloscope.