We have the function \( f(x) = \log(x^9) \). Using the properties of logarithms, we can simplify the function:
\[
\log(x^9) = 9 \log(x)
\]
Now, we take the derivative of this with respect to \( x \):
\[
\frac{d}{dx} \left( 9 \log(x) \right) = 9 \times \frac{1}{x}
\]
Thus, the derivative is:
\[
\frac{9}{x}
\]
Therefore, the correct answer is \( \frac{9}{x} \), which corresponds to option (C).