The correct answer is A. cost function.
A cost function is a mathematical expression that describes the relationship between the cost of producing a product or service and the level of activity associated with that production. The cost function is typically expressed as a linear equation, where the dependent variable is the cost and the independent variable is the level of activity.
The cost function is an important tool for businesses, as it allows them to estimate the costs associated with different levels of production. This information can be used to make decisions about pricing, production levels, and other aspects of the business.
The other options are incorrect because they do not describe the relationship between cost and level of activity. A revenue function is a mathematical expression that describes the relationship between the revenue generated by a product or service and the level of activity associated with that production. A unit function is a mathematical expression that describes the relationship between the output of a process and the input to that process. A relative function is a mathematical expression that describes the relationship between two variables, where the value of one variable is expressed as a proportion of the value of the other variable.