Establishing post offices is, and always has been a federal responsibility, you can tell by the fact that it is called the "United States Post Office." Making treaties is an aspect of foreign policy, which the states have nothing to do with. Public schools and education is stated as a responsibility of the states. So that leaves taxes. Taxes are required for any branch of government to run. This means that the state has to collect taxes to run itself, and the federal government also needs to collect taxes.
The answer is C., Collecting taxes.