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Answer: Least Count
Absolute uncertainty is the smallest possible error in a measurement, which comes from the least count of the instrument. The least count is the smallest value an instrument can measure.
Least count is the correct answer because it tells us the smallest step an instrument can take, which sets the limit for how precise a measurement can be.
Why the other option don’t fit the answer?
Accuracy is about how close a measurement is to the real value.
Fractional uncertainty and percentage uncertainty compare uncertainty to the total measurement, but they don’t define absolute uncertainty.