Which of the following is an indicator of egg quality?

Shell cleanliness is a significant indicator of egg quality because it directly affects both the safety and the quality of the eggs. A clean shell helps prevent the penetration of bacteria and other contaminants that could compromise the integrity of the egg within. When eggs are produced, they can sometimes have dirt, manure, or other residues on their shells, which can be a breeding ground for pathogens. Clean shells are also generally an indicator of proper handling and storage practices, suggesting that the eggs have been processed under sanitary conditions.

In contrast, the other options, such as the shape of the carton, color of the label, and type of carton material, do not impact the intrinsic quality of the eggs themselves. These aspects might contribute to consumer marketing or perception but do not provide direct information related to the edibility or quality of the eggs inside. Therefore, while they may be important for packaging and branding, shell cleanliness remains the most direct indicator of how safe and high-quality the eggs are.