Final answer:
Taxes imposed by Congress must apply uniformly throughout the U.S., according to the U.S. Constitution, and the 16th Amendment allows Congress to levy a federal income tax without apportioning it among the states.
Step-by-step explanation:
In regards to the taxing powers of the federal government, the U.S. Constitution stipulates that: a. taxes imposed by Congress must apply uniformly throughout the U.S. This is supported by Section 8 Clause 1 of the Constitution, which states that all Duties, Imposts, and Excises shall be uniform across the United States. It is important to note that, while direct taxes on income were initially prohibited, the 16th Amendment significantly changed this by authorizing Congress to establish a federal income tax without apportioning it among the states or basing it on any census or enumeration. Therefore, the answer to the student's question is (a).