Acurrency is a system of money created and used by
a. an individual or a group of people.
b. all people who live in the same era.
c. a nation or region of the world.
d. a town, city, or state within a region

I believe the answer is: c. a nation or region of the world.

A currency system refers to a system which only allow a specific currency that can be used as a medium of exchange for the transactions that happen in a country. Each nations across the world commonly have their own currency, and the value of their currency could either go down or up depending on their performance in the international trade.

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