In this scenario, the independent variable is the factor that you can control or choose without being influenced by other variables in the situation. Since you need 24 tea bags, the independent variable here would be the quantity you decide to choose in relation to the boxes of tea bags available.
Therefore, the correct answer would be:
E. The number of boxes of tea bags you need
This is because the quantity of boxes you need will determine the total number of tea bags and potentially influence the cost, but the number of tea bags you need (24) is a fixed requirement.