The correct answer is B.
Federalism is a sort of goverment which combines a supreme central goverment and several regional subgoverments which are subordinated to the first one.
In the United States, under the supreme authority of the US federal goverment, there are goverments at the state level, which have to follow the legislation designed and enforced by the central power. The US Constitution describes which are the powers reserved to each goverment level.