The question says to take the uncertainty in reading the current and voltage for instance, to be the last or fluctuating digit?..what is the last or flutuating digit? what does it mean?
When you plug your voltmeter into the circuit, perhaps the display is steady, and shows three decimal places. Then the uncertainty is 0.001V.
Or perhaps the display keeps changing. The first digit after the decimal place is steady, but the second one keeps flipping between 7 and 8, eg, your reading keeps flipping between 2.97V and 2.98V. Then the uncertainty is the fluctuating (changing back and forth) digit, ie, 0.01V.