Which of the following is the best instrument to use for checking the keying waveform of a CW transmitter?

An oscilloscope is an instrument designed to display voltage amplitude over time on a two-dimensional screen. This makes it the perfect tool to visualize the exact shape of a keying waveform, allowing you to check the rise and fall times of a Continuous Wave (CW) signal to ensure it won't cause key clicks. The distractors serve different purposes and cannot display a wave shape. Field strength meters and wavemeters measure overall RF amplitude or frequency, not the rapid changes over time. A sidetone monitor simply provides an audio tone for the operator to hear their Morse code, but it reveals nothing about the actual electrical characteristics of the transmitted RF envelope.