Decision variables:

A. tell how much or how many of something to produce, invest, purchase, hire, etc.
B. represent the values of the constraints.
C. measure the objective function.
D. must exist for each constraint.

The correct answer is A. tell how much or how many of something to produce, invest, purchase, hire, etc.

Explanation:

With the decision variables we represent the questions that we have and that motivate the formulation of the optimization problem. These questions must have a numerical answer, which implies a specific action and this action must be under our control.