So your answer is B: The output changes to 1 at Vref / (2^bits) + 1uV.
Say hello to your personal definition of "best fit".For everyone else, "best fit" happens when ADC counts are interpreted as being at the midpoint in a bucket.
Cutoff = Vref / (2^bits)For your example...Vref = 5 voltsbits = 10So...Cutoff = 5 / (2^10) = 0.0048828125
It is possible to know how to do something in practice without knowing the theory. The method by which aircraft wings generate lift is a good example.
The aircraft wings generate upward lift owing to the very universal law of the fluid mechanics.