The answer is "The congress"
In this case, the complete sentence would read: "Under the separation of powers principle, laws can only be made by the congress"
Here's a quick explanation on this:
The National Congress is a political institution that exercises the Legislative Power. It is composed of the House of Representatives and the Senate (formed by the senators). The main activities of the congressmen are related to the functions of legislation (which are the elaboration of new laws) and the supervision of the other powers.