Answer: A. a downward-sloping labor demand curve.
Step-by-step explanation:
The labor demand curve is plotted with the quantity of labor demanded vs the real wages paid to labor. In a firm that is producing in a market with a diminishing marginal product of labor, the demand curve will be downward sloping to reflect that the more labor that a company has, the less it pays them.
This is because the extra labor is bringing in less additional revenue and so will need to be paid accordingly to reflect that as more labor is hired, the output decreases.