Answer:
In a Cathode Ray Oscilloscope (CRO), the display of a sine wave is achieved by applying a sawtooth waveform to the horizontal plates.

Explanation:

The Cathode Ray Oscilloscope (CRO) is an electronic test instrument used to visualize electrical signals. It consists of a cathode ray tube (CRT) and various control circuits.

The horizontal deflection system of a CRO is responsible for sweeping the electron beam horizontally across the screen. This horizontal sweep is achieved by applying a voltage waveform to the horizontal plates, which deflects the electron beam sideways.

Types of Waveforms:

There are different types of waveforms that can be applied to the horizontal plates of a CRO, such as:

- Sinusoidal waveform: A sinusoidal waveform applied to the horizontal plates would result in a display of a sine wave on the screen.

- Square waveform: A square waveform applied to the horizontal plates would result in a display of a square wave on the screen.

- Sawtooth waveform: A sawtooth waveform applied to the horizontal plates would result in a display of a sawtooth wave on the screen. This waveform is commonly used to display a sine wave on a CRO.

Why Sawtooth Waveform is Used:

The sawtooth waveform is commonly used for displaying a sine wave on a CRO because it provides a linear and constant sweep across the screen. This linear sweep ensures that each point of the sine wave is displayed accurately and uniformly across the screen.

The sawtooth waveform is generated by a ramp generator circuit, which produces a linearly increasing voltage. This voltage is then applied to the horizontal plates of the CRO, causing the electron beam to sweep horizontally across the screen at a constant rate.

By adjusting the sweep rate and the amplitude of the sawtooth waveform, the frequency and amplitude of the displayed sine wave can be controlled. This allows for accurate measurement and analysis of the input signal.

Therefore, the correct answer is option 'C' - Sawtooth waveform.