Which modern-day country is home to the ancient city of Serdica?

The ancient city of Serdica is located in present-day Bulgaria. Historically, Serdica was an important city in the Roman Empire and is situated in the area that corresponds to the modern capital, Sofia. This location has been significant for various reasons, including its strategic position on trade routes and its role as a center of administrative and military power in the region during the Roman and Byzantine periods. The ruins of Serdica can still be seen today, showcasing its rich historical heritage and importance in ancient times. This context emphasizes why Bulgaria is the correct answer, as the other countries represented do not have historical claims to the site of Serdica.