Final answer:
The Legislative branch's duty is to write the laws, while other responsibilities like approving treaties and declaring war belong to the Executive branch.
Step-by-step explanation:
The duty of the Legislative branch is to write the laws.
The other options - approving treaties, declaring war, and proposing laws - are all duties of the Legislative branch. However, it is the Executive branch that has the power to approve treaties and declare war.
For example, the President of the United States, who is part of the Executive branch, has the authority to negotiate and sign treaties with other countries, which are then subject to approval by the Senate, a part of the Legislative branch.
Therefore, the correct option is OC Write the laws, as it is not a duty of the Legislative branch.
Learn more about Legislative branch duties