Which of the following is not a grade for apples?

The selection of "US" as the option that is not a grade for apples is correct because it refers to a classification system rather than a specific grade. In the context of apple grading, grades such as Grade A, Grade B, and Grade C refer to quality levels determined by factors like size, color, and blemishes.

Grade A typically denotes the highest quality apples that are uniform in size and color, with minimal imperfections. Grade B apples may have minor defects, while Grade C apples are of lower quality and may have more significant blemishes or size variations.

In contrast, "US" does not signify a specific quality or grade but could refer more broadly to the United States Department of Agriculture (USDA), which sets the grading standards. Therefore, it does not fit within the established grading classifications for apples.