in the long run when the average cost is rising, marginal cost must be.

Option 1 -> If MC equals AC, the average cost would be at its minimum point (neither rising nor falling).

Option 2 -> If MC is less than AC, each additional unit costs less than the average, pulling the average down, so AC would be falling.

Option 3 -> If MC is greater than AC, each additional unit costs more than the average, pulling the average up, so AC would be rising.

Option 4 -> MC can be rising, falling, or constant when AC is rising; the direction of MC doesn't determine whether AC rises.

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Hence, Option 3: Greater than the average cost -> When average cost is rising, the marginal cost must be greater than the average cost. This is a fundamental relationship in cost theory: the marginal value pulls the average in its direction. When you add units that cost more than the current average (MC > AC), the average increases. Conversely, when MC < AC, the average decreases. The two curves intersect at the minimum point of AC where MC = AC. -> correct