Nonnegativity constraints ensure that
a. the solution to the problem will contain only nonnegative values for the decision variables.
b. the problem modeling includes only nonnegative values in the constraints.
c. the objective function of the problem always returns maximum quantities.
d. there are no inequalities in the constraints.

Option 'a' is correct.

Step-by-step explanation:

Since non negativity constraints ensure that the solution to the problem will contain only non negative values for the decision variables.

Like if the decision variables are x, y.

Then if x>0, y>0 it means the solution must be either positive or 0.

It only contains the non negative values.

Hence, option 'a' is correct.