An objective function is the function of a linear programming model that is desired to be maximized or minimized. As also understood from the information given, the correct answer is A.

Information related to the other terms in the options as follows:

An optimal solution is a feasible solution that reaches the most favorable value of the objective function.

A mathematical model is an abstraction of the analyzed system using mathematical expressions.

Linear models consist of constraints and an objective in the form of a linear expression.

A non-negativity constraint requires the value x to be greater or equal to zero.