Which category does not belong to the USDA grading system for vegetables?

The correct answer is that "U.S. No.1" does not belong to the USDA grading system for vegetables as it actually represents a category within that system. The USDA grading system categorizes vegetables into various qualities based on specific criteria, such as size, shape, and ripeness.

"U.S. Extra Fancy" is the highest quality grade, ensuring superior quality and appearance. On the other hand, "U.S. No.2" and "U.S. No.3" grades are lower categories that indicate decreasing quality standards, with "U.S. No.2" reflecting good quality and "U.S. No.3" grade often showing more defects.

Thus, each of the other choices represents a viable category in the USDA grading system, where "U.S. No.1" is a recognized grade, while the assertion that it does not belong is inaccurate. This understanding emphasizes the importance of knowing the grading standards when evaluating or purchasing vegetables.