The correct answer is - currency.
The objects or group of objects that have been widely agreed upon to be used a s form of exchange is called currency. The currency is a very useful method for exchanging things, and it has been widely accepted around the world, with some currencies being globally accepted, while some only regionally, or locally.
The reason why the currencies have been introduced is so that the two or more sides in the process of exchange have the same means and valuations, thus making the exchange very easy. Some of the strongest currencies in the world are the American Dollar, Euro, British Pound, Swiss Frank...