The correct answer is "The Senate when it ratifies treaties and accepts or rejects diplomatic appointments."
Foreign Relations conducted by the President can be "checked" by another branch. The choice that is an example of that is "The Senate when it ratifies treaties and accepts or rejects diplomatic appointments."
The President of the United States is the most important political figure of the federal government. He is the chief diplomat, regarding foreign affairs. However, the US Congress, the Senate, has the power to ratify treaties. For instance, that was the case of the ratification in the US Congress of the USMCA, the United States, México, and Canada Agreement, that substituted NAFTA, the North American Foreign Trade Agreement.