Concept:
The rms speed of an ideal gas is given by:
\[
v_{rms} = \sqrt{\frac{3RT}{M}}
\]
Thus:
\[
v_{rms} \propto \sqrt{T}
\]
This means:
\[
\frac{v_2}{v_1} = \sqrt{\frac{T_2}{T_1}}
\]
Therefore, when rms speed changes, temperature changes as the square of speed ratio.
Step 1: Write the proportional relation.
Initially:
\[
v_1 = x
\]
at temperature:
\[
T_1 = T
\]
Finally:
\[
v_2 = 3x
\]
Required temperature:
\[
T_2 = ?
\]
Using:
\[
\frac{v_2}{v_1}
=
\sqrt{\frac{T_2}{T_1}}
\]
Step 2: Substitute the values.
\[
\frac{3x}{x}
=
\sqrt{\frac{T_2}{T}}
\]
\[
3 = \sqrt{\frac{T_2}{T}}
\]
Squaring both sides:
\[
9 = \frac{T_2}{T}
\]
Thus:
\[
T_2 = 9T
\]
Step 3: Write the final answer.
Hence, required temperature is:
\[
\boxed{9T}
\]
Therefore, the correct option is:
\[
\boxed{(C)}
\]