Final answer:
The best-completing phrase for the diagram on Native American tribes' recent gains is likely related to the ability to renegotiate treaties of sovereignty, reflecting the improvement in tribal autonomy and governance.
Step-by-step explanation:
The phrase that best completes the diagram regarding recent gains for Native American tribes likely relates to policies that have helped to strengthen their sovereignty and governance. Notably, the ability to renegotiate treaties stands out as a key milestone, reflecting a restoration of agency and respect for tribal sovereignty. Other notable gains include the strengthening of land claims which has resulted in reservation lands being placed back into the hands of tribal leadership and bolstered self-governance.