The correct answer is: D. slope coefficient.
The slope coefficient is the coefficient of the independent variable in a linear equation. It represents the amount by which the dependent variable changes for every unit change in the independent variable. In the context of the question, the slope coefficient represents the amount by which total cost changes for every unit change in the level of activity.
The activity coefficient is a measure of how strongly a solute interacts with the solvent. It is a dimensionless number that is always between 0 and 1. A value of 1 indicates that the solute and solvent interact equally, while a value of 0 indicates that the solute and solvent do not interact at all.
The cost coefficient is a measure of the cost of producing one unit of output. It is a per-unit cost that is used to calculate the total cost of production.
The change coefficient is a measure of how much a dependent variable changes in response to a change in an independent variable. It is calculated by dividing the change in the dependent variable by the change in the independent variable.
In conclusion, the slope coefficient is the correct answer to the question. It represents the amount by which total cost changes for every unit change in the level of activity.