The best description of the nature of reserved powers is:
B. States can engage in any activity unless there is some sort of prohibition against it.
Reserved powers refer to those powers that are not delegated to the national government by the Constitution nor prohibited to the states, allowing states to exercise a wide range of legislative and regulatory responsibilities. This aligns with the 10th Amendment, which emphasizes that powers not given to the federal government nor prohibited to the states are reserved for the states or the people.