The correct answer is option C. i.e. 4%
The total thickness of the stack of sheets is measured to be (0.80 ± 0.02) mm.
The maximum true thickness will be (0.80 + 0.02) mm. = 1.00 mm
Ans, the maximum uncertainity will be (0.20 + 0.02) mm = +/-0.04 mm
Thus, the four sheets true thickness will be 1.00 mm +/-0.04 mm
Thus, single sheet thickness will be 0.250 mm +/-0.01 mm
Thus, percentage error = (0.01/0.25) * 100 = 4%