The numbers are \(9.99 \text{m}\) (2 decimal places) and \(0.0099 \text{m}\) (4 decimal places). For subtraction, the result should have the same number of decimal places as the number with the fewest decimal places (2 in this case). \(9.9900 - 0.0099 = 9.9801\). Rounding \(9.9801\) to 2 decimal places gives \(9.98 \text{m}\).

The mean of the values is \[\frac{3.33 + 3.32 + 3.34 + 3.33 + 3.32}{5} = 3.328 \text{cm}\]. Since the measurements are up to two decimal places, the rounded final result should also have two decimal places: 3.33 cm.

The most accurate value is the mean value of the observations. Mean length = \[\frac{3.29 + 3.28 + 3.29 + 3.31 + 3.28}{5} = \frac{16.45}{5} = 3.29 \text{cm}\].

Statements A and B are correct properties of measurements. Statement C is incorrect because the described quantity is the absolute error, not the relative error.

By taking systematic errors into account, the relative error can be written with signs: \( \frac{\Delta X}{X} = \pm \frac{\Delta A}{A} \pm 2\frac{\Delta B}{B} \pm 3\frac{\Delta C}{C} \). The minimum absolute error is \( |1 + 4 - 9|\% = 4\% \), so 4% is a possible value.

In addition, the final result must be rounded off to the least number of decimal places of the terms. Here, \(227.2\text{ g}\) has one decimal place, so the sum \(663.821\text{ g}\) is rounded to \(663.8\text{ g}\).