Why is the Necessary and Proper Clause a source of ongoing debate?

Congress cannot agree on how broad its implied powers should be.

The wording of the clause is often undergoing revision.

The clause deals with specified powers that are ambiguous and misinterpreted.

The clause establishes reserved rights, which vary from state to state.

The correct answer here is the first option.

There are few types of power granted by the US constitution and two of those are enumerated or expressed powers which are specifically granted by the constitution and there are implied powers that is powers which are to not explicitly stated in the constitution but are implied by those that are. Necessary and Proper Clause is problematic as it allows the Congress to pass laws which will make it possible for the expressed powers to be carried out. There is a debate just how much power is implied.

Why is the Necessary and Proper Clause a source of ongoing debate?

The clause deals with powers that are ambiguous and misinterpreted.

The "necessary and proper" clause is included in Article I, Section 8, of the United States Constitution. After enumerating a number of the powers of Congress, including borrowing money, coining money, regulating commerce, etc, Section 8 of Article I closes with by saying Congress shall have power "to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof." That's a quite broad and sweeping statement. Political leaders have differed in how they would interpret and apply that sort of authority.