_______________ lines on a CIE diagram are lines in the x-y plane along which all colors are indistinguishable.

Color confusion lines on a CIE diagram represent the paths along which various colors cannot be distinguished from one another by an observer. This concept is essential in understanding color perception, as it highlights the limitations of human vision in differentiating certain colors under specific conditions. These lines effectively illustrate how certain pairs of colors appear similar or identical to the human eye, depending on the lighting and the observer’s visual system.

The idea of color confusion lines emphasizes how various colors map within the color space defined in the CIE diagram, which is a tool used for color representation in a standardized manner. The distinction of colors along these lines indicates areas of color overlap where visual perception does not register any difference, leading to confusion.

The other choices, while related to color theory, do not specifically capture this phenomenon as accurately as color confusion lines do. Color discrimination lines would refer to areas where colors can be distinctly identified, whereas chromaticity lines generally describe paths of constant chromaticity rather than areas of indistinguishability. Transection lines do not have a well-established definition in the context of the CIE diagram. Therefore, color confusion lines are the most appropriate term to describe the lines along which colors become indistinguishable in the diagram.