The best description of the nature of reserved powers is:
States can engage in any activity unless there is some sort of prohibition against it.
This option reflects the concept of reserved powers, which are those powers that are not delegated to the federal government by the Constitution and are instead reserved for the states. According to the Tenth Amendment, powers not expressed in the Constitution are left to the states or the people, allowing states to exercise authority in a wide range of matters unless specifically restricted.