The standard deviation of the set of analytical results i.e. 1.72, 1.74, 1.68, 1.71, 1.69 gm is

Mean = 1.708. Squared deviations: 0.000144 + 0.001024 + 0.000784 + 0.000004 + 0.000324 = 0.00228. Dividing by $n = 5$ gives 0.000456, whose square root is 0.02135. The $n-1$ value, 0.0239, is not offered; 1.708 is the mean.