Step 1: Arrange the observations.
Without \(x\),
\[
4,6,8,9,10,11,15,17.
\]
To have a mode, one observation must repeat.
Among the options, choosing
\[
x=10
\]
makes 10 occur twice.
Step 2: Find the median.
The ordered data become
\[
4,6,8,9,10,10,11,15,17.
\]
Since there are 9 observations, the median is the 5th observation.
\[
\text{Median}=10.
\]
Step 3: Find the mean.
Sum of first eight observations:
\[
4+6+8+9+10+11+15+17=80.
\]
Adding \(x=10\),
\[
\text{Total}=90.
\]
\[
\text{Mean}
=
\frac{90}{9}
=
10.
\]
Thus
\[
\text{Mean}=\text{Median}=\text{Mode}=10.
\]
Hence
\[
\boxed{x=10}.
\]