Which EU country adopted the euro in 2015?

Lithuania adopted the euro as its official currency on January 1, 2015. This transition marked a significant milestone for the country, as it replaced the Lithuanian litas with the euro, aligning itself more closely with the European Union's economic policies and strengthening its integration into the Eurozone. The adoption of the euro not only facilitated trade and investment but also aimed to enhance economic stability and foster deeper ties within the EU.

Prior to Lithuania, the other countries listed had joined the Eurozone at different times: Estonia adopted the euro in 2011, Latvia in 2014, and Slovakia in 2009. Each of these countries similarly aimed to solidify their economic participation within the European Union through the euro, but Lithuania's adoption specifically occurred in 2015, making it the correct choice in this context.