To solve this problem, we need to reflect point $C(3, -9)$ over the line $y = x$. The rule for reflecting a point $(x, y)$ over the line $y = x$ is to swap the coordinates, meaning the point $(x, y)$ becomes $(y, x)$.

For point $C(3, -9)$, reflecting over the line $y = x$ gives us:

$C' = (-9, 3)$

Thus, the coordinates of the reflected point are $(-9, 3)$.

The correct answer is B. $(-9, 3)$.

Would you like further explanation or details? Here are five related questions to enhance your understanding:

1. How do you reflect a point over the line $y = -x$?
2. What is the rule for reflecting points over the x-axis or y-axis?
3. How would you calculate the reflection of a point over a different line, such as $y = 2x + 1$?
4. How can you determine if two figures are congruent after a reflection?
5. What are some practical applications of geometric reflections in real-world scenarios?

Tip: Remember that reflecting over the line $y = x$ always involves swapping the x- and y-coordinates.