Answer:
The correct answer is option a.
Step-by-step explanation:
In a perfectly competitive labor market, after some point, the marginal revenue product derived from hiring an additional worker starts declining. This causes the marginal revenue curve to slope downward after a certain point.
This happens because of diminishing marginal returns. The law of diminishing marginal returns states that keeping other things constant if we keep increasing a variable factor, after certain the marginal returns from each additional unit will start declining.