Aeuro is:
a. a greek sandwich consisting mostly of beef.
c. an industry recently established in europe.
b. a regional trade organization in europe.
d. european currency.

D. European currency

Most of the EU (European Union) use the euro but 9 countries: Bulgaria, Croatia, Czech Republic, Denmark,Hungary, Poland, Romania, Sweden, and the United Kingdom use their own currency. 
Most countries however do use the euro after joining the EU, like France dropped using the franc and started using the euro.