The correct option is E It failed to mention water rights to be enjoyed by the reservation's inhabitants.
"In Winters v. United States (1908), the Supreme Court held that the right to use waters flowing through or adjacent to the Fort Belknap Indian Reservation was reserved to American Indians by the treaty establishing the reservation. Although this treaty did not mention water rights, the Court ruled..." - in the given sentences extracted from the passage, the words 'treaty establishing the reservation' refers to the treaty establishing the Fort Belknap reservation and the very next sentence explicitly states that water rights were not mentioned in the treaty. Thus, the above extract supports option D as the answer.