Final answer:
The body responsible for deciding what is legal and illegal is the Legislative Branch, specifically Congress, which is composed of the House of Representatives and the Senate.
Step-by-step explanation:
The body that decides what is legal and illegal is the Legislative Branch. According to Article 1 of the Constitution, only Congress, which is a bicameral legislature consisting of the House of Representatives and the Senate, can make laws. While the Executive Branch, led by the President, is responsible for enforcing the laws passed by Congress, and the Judicial Branch, composed of courts like the Supreme Court, interprets laws and determines their constitutionality, it is the Legislative Branch that actually creates and passes legislation that defines what is legal and what is not.