Agreement: Choosing the Correct Preposition

The sentence requires a preposition to complete the phrase "entered _____ an agreement". The standard and idiomatic phrase in English is "to enter into an agreement", which means to formally start or become involved in an agreement.

Choosing the Right Preposition

• Into: This preposition is used to indicate movement or transition from one state or condition to another. In the context of agreements, "enter into" signifies the act of formally establishing the agreement. This fits the sentence perfectly: "The two groups entered into an agreement..."
• At: Typically used to indicate a specific point in time or location, or a specific price/rate. It does not fit the context of forming an agreement.
• In: Often used to indicate being inside something or belonging to a group. While one can be "in" an agreement, the act of *starting* or *forming* it uses "enter into".
• Unto: An archaic form of "to" or "into", rarely used in modern English, especially in this context.

Therefore, the correct preposition to signify the formal commencement of the agreement is "into".