If a total cost curve is plotted, marginal cost can be illustrated by the A. slope of the tangent to the curve B. u-shaped curve cutting the total cost curve at its lowest point C. straight line from the origin to the mid-point of the curve D. straight line cutting the curve at its highest point

Step 1: Recall the definition of marginal cost. Marginal cost (MC) is the additional cost of producing one more unit of output, given by MC = (dTC)/(dQ), where TC is total cost and Q is quantity. This is the slope of the tangent line to the total cost curve at any point Q. Step 2: Identify the graphical representation. The slope of the tangent to the TC curve directly illustrates MC at each output level, matching option A. Step 3: Evaluate other options. Option B: The MC curve is U-shaped and intersects the average total cost (ATC) curve at its minimum, but it does not "cut" the TC curve at a lowest point (TC has no lowest point beyond Q=0). Option C: A straight line from the origin to a point (Q, TC) on the curve has slope (TC)/(Q) = ATC, not MC. Option D: No economic interpretation matches a straight line cutting TC at its highest point. The correct answer is A. A