A perfectly competitive firm's supply curve is its: a) marginal cost curve above the minimum of its average fixed cost

The answer is b).

In a competitive market, all firms are price takers, therefore, for all firms the marginal revenue from each unit produced is equal to the market price. Now for firms to maximize profit, they produce until the marginal revenue is equal to the marginal cost. Since marginal revenue is equal to price for levels of output, the firm's level of output is one such that marginal cost is equal to price. Therefore, at any given price, the quantity supplied could be traced along the marginal cost curve until marginal cost is equal to price. In other words, the supply curve is the marginal cost curve.

However, not the entire marginal cost curve is relevant. This is because if the average variable cost is above the price, then the firm's revenue is negative in the short run. In this case, the firm is better off shutting down rater than producing. Hence, the relevant portion of the marginal cost curve is the portion that is above the minimum average variable cost.