The correct answer is: A. Peak value to r.m.s. value.
The form factor is a dimensionless quantity that is used to compare the shape of two waveforms. It is defined as the ratio of the peak value of the waveform to the root-mean-square (rms) value of the waveform.
The rms value of a waveform is the square root of the average of the squared values of the waveform. It is a measure of the effective value of the waveform, which is the value that would produce the same heating effect in a resistor as a direct current of the same magnitude.
The peak value of a waveform is the maximum value of the waveform. It is the value that the waveform reaches at its highest point.
The form factor is important in many applications, such as power distribution and electrical engineering. It is used to determine the size of conductors and other components that are used to carry alternating current (AC) power.
The other options are incorrect because they do not represent the correct definition of the form factor.
Option B: The r.m.s. value to average value is the definition of the crest factor.
Option C: The average value to r.m.s. value is the definition of the power factor.
Option D: None of the above is the correct answer.