This is quite a trick question indeed. Isaac Stevens had been appointed as Governor of the Washington Territory by President Franklin Pierce. He was also named Superintendent of Indian Affairs and was thus directly in charge of any official dealings with Native American tribes.
He did force several tribes to sign treaties with the federal government to cede their lands to it. Although he acted autocratically and in a very authoritarian manner, he did it in the capacity of the positions described above and as a representative of the territorial government has was its representative therefore the correct answer is “The territorial government” as he did not act this way out of a private initiative.
