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
$MC = \frac{dTC}{dQ},$
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 $\frac{TC}{Q} = ATC$, not $MC$.
• Option D: No economic interpretation matches a straight line cutting $TC$ at its highest point.

The correct answer is A.
A