Answer:
A formal agreement
Step-by-step explanation:
The word compact has several meanings, however, based on the context clues that indicate that the text comes from a Supreme Court case (which tends to be formal) and the case is about determining to what extent The Tongue and Powder Rivers should benefit Wyoming and North Dakota, we can infer that compact refers to a formal agreement or contract. It is a contract that can "allow" someone to do something (in this case, Wyoming), and that afford protection to someone or something (in this case, both states).