In cylindrical coordinates, $z$ simply denotes the $z$-coordinate of a point.  Thus, for a fixed $z$-coordinate $c,$ all the points lie on a plane that is parallel to the $xy$-plane.  The answer is $\boxed{\text{(C)}}.$

[asy]
import three;
import solids;

size(200);
currentprojection = perspective(6,3,2);
currentlight = (1,0,1);
real theta = 120;

draw((-2,0,0)--(2,0,0));
draw((0,-2,0)--(0,2,0));
draw(surface((1,1,0.5)--(1,-1,0.5)--(-1,-1,0.5)--(-1,1,0.5)--cycle),gray(0.99));
draw((0,0,-2)--(0,0,0.2));
draw((0,0,0.5)--(0,0,2));

label("$x$", (2,0,0), SW);
label("$y$", (0,2,0), E);
label("$z$", (0,0,2), N);
label("$z = c$", (-1,1,0.5), E);
[/asy]
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
For a constant $c,$ in cylindrical coordinates $(r,\theta,z),$ find the shape described by the equation
\[z = c.\](A) Line
(B) Circle
(C) Plane
(D) Sphere
(E) Cylinder
(F) Cone

Enter the letter of the correct option.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\text{(C)}
For a constant $c,$ in cylindrical coordinates $(r,\theta,z),$ find the shape described by the equation
\[z = c.\](A) Line
(B) Circle
(C) Plane
(D) Sphere
(E) Cylinder
(F) Cone

Enter the letter of the correct option.
test