The correct answer is option 'D', which states that the quantity that can be made out by observation in a CRO (Cathode Ray Oscilloscope) is the peak-peak value of the sine wave. Let's understand why this is the correct answer.

Explanation:

1. Cathode Ray Oscilloscope (CRO):
A Cathode Ray Oscilloscope is an electronic instrument used to visualize and analyze the waveform of electrical signals. It displays the signal on a screen by deflecting an electron beam generated by a cathode ray tube (CRT). CROs are extensively used in various fields, including electronics, telecommunications, medicine, and research.

2. Sinusoidal Waveform:
A sinusoidal waveform is a type of waveform that represents a sine function. It is characterized by its frequency, amplitude, and phase. In a CRO, a sinusoidal waveform can be displayed by applying the input signal to the vertical deflection plates of the CRT.

3. Observing the Sinusoidal Waveform:
When a sinusoidal waveform is displayed on the screen of a CRO, several quantities can be observed or measured. These quantities provide valuable information about the waveform. Let's discuss each option mentioned in the question and understand why the correct answer is the peak-peak value.

a) RMS value of the sine wave:
The RMS (Root Mean Square) value of a sinusoidal waveform represents the effective or average value of the waveform. It is calculated by taking the square root of the average of the squared values of the waveform. However, in a CRO, it is not possible to directly measure the RMS value of the waveform by observation. Therefore, this option is not the correct answer.

b) Average value of the sine wave:
The average value of a sinusoidal waveform represents the average of all the instantaneous values of the waveform over a complete cycle. Similar to the RMS value, it is not possible to directly measure the average value of the waveform by observation in a CRO. Hence, this option is also not the correct answer.

c) Form factor of the sine wave:
The form factor of a sinusoidal waveform is the ratio of the RMS value to the average value of the waveform. It provides information about the shape or form of the waveform. However, as mentioned earlier, both the RMS value and the average value cannot be directly measured in a CRO. Therefore, this option is not the correct answer.

d) Peak-peak value of the sine wave:
The peak-peak value of a sinusoidal waveform represents the difference between its maximum positive peak and its maximum negative peak. In other words, it is the total span or amplitude of the waveform from its highest point to its lowest point. This quantity can be easily observed and measured by directly looking at the waveform displayed on the screen of a CRO. Hence, this option is the correct answer.

Conclusion:
In a CRO, the quantity that can be made out by observation when a sinusoidal waveform is displayed is the peak-peak value of the sine wave. This value represents the amplitude or span of the waveform from its highest point to its lowest point.