Answer: C. the units of measurement on both sides of the constraint match.
Explanation:
A constraint simply refers to the condition with regards to an optimization problem which must be satisfied by the solution.
When formulating a constraint, care must be taken to ensure that the units of measurement on both sides of the constraint match.
It should be noted that it's not necessary for all the objective function coefficients to be included. Also, there must be inequalities in the expression. Therefore, the correct option is C.