Final answer:
In a long-run equilibrium for a perfectly competitive industry, price (P) equals both marginal revenue (MR) and marginal cost (MC), as well as average variable cost (AVC).
Step-by-step explanation:
In a long-run equilibrium for a perfectly competitive industry, the conditions are such that price (P) equals both marginal revenue (MR) and marginal cost (MC), as well as average variable cost (AVC).
Therefore, the correct option is c. P = MR = MC = AVC.