The most accurate answer would be:
C. These treaties were often not rectified by the US Senate.
In the context of U.S. history, treaties between Indigenous peoples and the federal government require ratification by the Senate to be enforceable. Many treaties made with Indigenous nations were either not ratified or were not honored by the government after being ratified, leading to a lack of enforceability. Other options do not directly address the treaty ratification process as governed by the U.S. Constitution.