The President meets with the Prime Minister of the United kingdom

The Supreme Court decides a case

Congress passes a new environmental law

Which constitutional principle do these examples illustrate?

This would most likely be separation of powers, or basically talking about how the powers in the US government are separated with the three branches of government. This is talking about how the President, or executive branch, deals with foreign things, Congress, or the legislative branch, can pass new laws, and the Supreme Court, or the judicial branch, can decide cases.