Concept:
Every firm, regardless of the market structure it operates in, maximises its profit at the output level where the extra revenue from selling one more unit equals the extra cost of producing that unit.
Explanation:
A firm under perfect competition is a price taker, so its Average Revenue (price) equals Marginal Revenue, both shown as a horizontal line at the market price. Even so, the firm still reaches its equilibrium (profit-maximising) output at the point where Marginal Revenue equals Marginal Cost, provided the Marginal Cost curve is rising through this point from below, which is the standard condition of profit maximisation used across all market structures. Simply having MC greater than AC, or MR greater than AC, or MR greater than AR does not pin down a unique output level, they are either always true past a certain output or, in the case of MR greater than AR, not even true for a perfectly competitive firm since MR equals AR throughout.
Final Answer:
Equilibrium output for a competitive firm is where MR = MC, so option D is correct.