In 3-dimensional space, the equation $x^2-8 x+12=0$ represents ....

The quadratic equation $x^2 - 8x + 12 = 0$ factors as $(x - 2)(x - 6) = 0$, yielding solutions $x = 2$ and $x = 6$.

In three-dimensional space, equations of the form $x = k$ represent planes parallel to the YZ-plane. Specifically, $x = 2$ and $x = 6$ define two distinct planes, both parallel to the YZ-plane and located at distances of 2 and 6 units from it along the positive X-axis.

Among the options, C) 2 planes parallel to YZ-plane correctly describes this configuration, as planes parallel to the XZ-plane would instead be given by equations of the form $y = k$.

The final answer is $\boxed{\text{C}}$.