Serbia contains part of the Iron Gates gorge on the Danube River.
The Iron Gates are a spectacular series of gorges and rapids where the Danube cuts through the Carpathian and Balkan mountains along the Serbia–Romania border. The name is often used for the wider gorge system as well as the narrowest passages.
The Iron Gate I Hydroelectric Power Station, completed in 1972, is jointly operated by Serbia and Romania. Its reservoir changed navigation and river levels, while the project also submerged or relocated several archaeological and historical sites.
Romania is a plausible answer because the gorge forms an international border and major facilities are shared. Serbia is the intended country because the question asks which landlocked country contains the Serbian side of the Iron Gates; Serbia’s most famous nearby archaeological site, Lepenski Vir, is also on the Danube.