What is a requirement applicable only to "C" grade eggs?

The requirement that is applicable only to "C" grade eggs is that they must be virtually clean. In the grading system for eggs, "C" grade reflects a lower quality compared to higher grades such as "A" or "B." For "C" grade eggs, cleanliness is a specific concern; they may contain dirt or other foreign material but should not be excessively dirty. This requirement underscores the importance of a minimum standard of cleanliness, even for lower-grade eggs.

In contrast, the other options relate to qualities that are generally required for higher grades of eggs or are not specific to "C" grade. Firm shell and medium size are characteristics more commonly associated with higher quality grades, emphasizing strength and uniformity in size respectively. Similarly, the requirement regarding blemishes is stricter for higher grade classifications, ensuring that eggs meet certain aesthetic and quality standards. Thus, the specification of being "virtually clean" is unique to "C" grade eggs and delineates the basic condition allowed for this classification.