Step 1: Concept
The derivative of an inverse function is given by the formula: $f'(x) = \frac{1}{g'(f(x))}$ or $g'(x) = \frac{1}{f'(g(x))}$.
Step 2: Meaning
We are given $g = f^{-1}$, which means $f$ and $g$ are inverses. If $g(5) = 6$, then by the property of inverses, $f(6) = 5$.
Step 3: Analysis
Using the derivative of the inverse function formula at $x = 5$ for the function $g$:
$g'(5) = \frac{1}{f'(g(5))}$.
Substitute the known values: $\frac{3}{4} = \frac{1}{f'(6)}$.
Rearranging to find $f'(6)$: $f'(6) = \frac{1}{3/4} = \frac{4}{3}$.
Step 4: Conclusion
The value of $f'(6)$ is $\frac{4}{3}$.
Final Answer: (D)