What is the required condition for the rear side marker light?

The required condition for the rear side marker light is that it should be securely attached, clean, and red in color. This requirement ensures that the light is properly positioned to be visible to other drivers, which is critical for safety on the road. A securely attached light helps prevent it from falling off during operation or being damaged, while cleanliness ensures that the light is functioning at its full brightness and can effectively alert other drivers to the vehicle's presence. The color red is specifically mandated for rear side marker lights, as it conforms to safety regulations that help standardize vehicle lighting and enhance visibility, particularly at night or in low-light conditions.

In contrast, other options suggest conditions that could compromise safety or violate regulations. For example, a cracked light that still works could be inadequate because it might not be sufficiently visible or could potentially break further. An indicator that must be a different color than red would not meet safety standards, as red is designated for specific functions. Lastly, a light that is not securely attached poses a significant hazard due to the risk of it becoming dislodged while the vehicle is in motion.