What is a treaty? ОА.
an informal agreement between two or more nations
B.
a formal agreement between two heads of state
O c.
an informal agreement between two heads of state
OD.
a formal agreement between two or more nations

OD.  A formal agreement between two or more nations.

Explanation:

A treaty is a formal agreement that is signed between two or more nations, according to the parties involved in that particular issue. It is a form or sign of a deal where the parties involved agreed to come to terms and signify the start of a peaceful co-operation between them.

The signing of treaties has been one of the most significant aspects of many events in the history of the world. Numerous wars and discords were solved with signing treaties, and also allows for peace and co-operation. These treaties contain pacts, deals, convention, and the participation of the involved parties in making peaceful efforts to progress, away from violent means.

Some of the more famous treaties in the world's history are The Treaty of Versailles in 1919, the Treaty of Paris in 1783, the Charter of the United Nations Organization in 1945, etc.