The equation given is:

$x^2 + z^2 = 9$

This is the equation of a circular cylinder. It represents a cylinder with radius 3 (since $9 = 3^2$) oriented along the y-axis (as the variable $y$ is absent from the equation, indicating no variation in that direction).

Correct options:

1. Surface Description: The correct option is D. circular cylinder.

2. Sketch the Surface: Among the four 3D plots shown (A, B, C, and D), D is the plot of a circular cylinder, as it depicts a surface that wraps around an axis (likely the y-axis), matching the given equation $x^2 + z^2 = 9$.

Would you like further explanation or details about cylindrical surfaces?

Here are 5 follow-up questions to expand on this:

1. How does the absence of the $y$ variable affect the shape of the surface?
2. What is the general form of a cylinder equation in 3D space?
3. Can the equation $x^2 + z^2 = r^2$ represent any other geometric shape in different contexts?
4. How would the graph change if the equation included $y^2$?
5. What are the distinguishing features of an elliptic cylinder?

Tip: When identifying 3D surfaces, always check for missing variables, as they often indicate an extrusion along the corresponding axis!