Ans: A
I must be true. If we add $9n$ to each of the data of the group $\{1-9n, 3-9n, 4-9n, 5-9n, 7-9n\}$, the new group becomes $\{1,3,4,5,7\}$.
Since adding a constant to each of the data of a group of number will not affect the value of the standard deviation, then the standard deviations of two group are the same.
By using a calculator, the standard deviation of $\{1,3,4,5,7\}$ is $2$.
Hence, $u=2$.
II may not be true. The median of the group of numbers is $4-9n$. If $n$ is a negative integer, then the median will be greater than $4$.
III is false. The range
$\begin{array}{cl}
= & (7-9n)-(1-9n) \\
= & 6 \text{ , which is not greater than $6$.}
\end{array}$
