Which of the following is one of the adjectives Mr. Brock used to illustrate leanness in a market hog?

The adjective "clean" is used by Mr. Brock to illustrate leanness in a market hog because it conveys the idea that the hog has a sleek appearance without excess fat. In livestock evaluation, a clean appearance indicates that there is minimal fleshiness around the shoulders, belly, and rear, which is an important characteristic for market readiness. A clean hog is typically thought to be more desirable as it is perceived to have higher meat yield with less waste.

The other terms, while they may describe various attributes, do not specifically refer to the quality of leanness. For example, "expressionless" does not have a clear correlation with physical traits in livestock, and "cut" or "dotting" do not convey the same visual representation of minimal fat as "clean" does. Thus, "clean" stands out as the appropriate choice to represent leanness in this context.