The correct answer is: Curvature.
The point of inflexion is the point on the total physical product curve where the curvature changes from concave to convex or vice versa. This means that the rate of change of the total physical product is increasing or decreasing at that point.
The other options are incorrect because they do not describe the point of inflexion. Option A, “Design”, is incorrect because the point of inflexion does not refer to the design of the total physical product curve. Option B, “Patterns”, is incorrect because the point of inflexion does not refer to the patterns of the total physical product curve. Option C, “None of the above”, is incorrect because the point of inflexion is a real and important concept in economics.