In a perfectly competitive market, each firm produces its last unit at the same marginal cost. So, the correct option is a.
Since firms in a perfectly competitive market are identical, their marginal cost is the same, and each firm produces its last unit at the point where the firm is equal to the marginal cost.
Marginal cost refers to the increase or decrease in the cost of producing one more unit or serving one more customer. It is also known as incremental cost. Marginal cost are based on production expenses that are variable or direct - labor, materials, and equipment. There is a marginal cost when there are changes in the total cost of production.
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