Which of the following is a valid concern when checking a rabbit's ears?

The valid concern when checking a rabbit's ears is primarily related to color. A rabbit's ear color can be an indicator of health, as any unusual discoloration may suggest issues such as infections or other health problems. Healthy ears are typically well-pigmented, and any changes in color, such as redness or excessive paleness, can warrant further investigation by a veterinarian.

While weight, height, and length are all important aspects to consider when assessing a rabbit's overall condition, they do not specifically pertain to the health of the ears in the same direct manner as ear color does. Thus, color serves as a more relevant marker for potential health concerns in a rabbit's ears.