Objective function of a linear programming problem is

a function to be optimized

The correct answer is (1) a function to be optimized.
In a linear programming problem, the objective function is a mathematical expression representing the quantity that the decision variables are aimed to either maximize or minimize. The goal of the linear programming problem is to find the values of the decision variables that optimize (maximize or minimize) the objective function while satisfying the given constraints. Therefore, the objective function is indeed the function that you are trying to optimize.