Final answer:
The true condition for a profit-maximizing firm in a perfectly competitive industry is where MR = MC, which is also expressed as P = MC because in perfect competition, MR equals the price. The correct answer is option C. MR=MC
Step-by-step explanation:
The condition that is TRUE for a profit-maximizing firm in a perfectly competitive industry is that the marginal cost (MC) is equal to marginal revenue (MR). A perfectly competitive firm's profit-maximizing choice occurs at the level of output where MR = MC. One critical aspect of this condition is that in perfect competition, marginal revenue is also equal to the price (MR = P), because each firm is a price taker and can sell as much as it wants at the market price.
The other options provided, such as ATC = AFC, MC = AVC, or MR = AFC, do not define the profit-maximizing condition for a perfectly competitive firm. Only when a firm produces at the quantity where P = MC, which is the same as MR = MC because MR equals the price in perfect competition, does it maximize profits. At this point, it can be assumed that no further profit can be made by increasing or decreasing production.