Answer:
D. the factor market
Step-by-step explanation:
Labor (along with land, capital, and entrepreneurship) is one of the "four factors of production."
Also, Dave isn't starting a business (so he's not an entrepreneur), nor is he in the product market (since he's not making products; just shelving, bagging, and/or scanning them at checkout). I suppose he might be participating in human capital in that he's building up "human capital" (i.e., building up his skills and experience), but I feel like "factor market" is better.