The Sava is the longest right-bank tributary of the Danube.
The river runs for about 990 kilometres from northwestern Slovenia through Croatia and along part of the Bosnia and Herzegovina border before entering Serbia. It joins the Danube at Belgrade, making the Serbian capital one of the few major cities situated at the confluence of two important European rivers.
The Sava begins with headwaters including the Sava Dolinka and Sava Bohinjka. Its basin drains a large part of the Dinaric Alps and includes tributaries such as the Drina, Bosna, Una, Kupa and Vrbas. The river is especially significant by water volume: it is the Danube’s largest tributary by discharge.
The key quiz distinction is “right-bank.” The Tisza is longer than the Sava and has a larger catchment area, but it enters the Danube from the left bank. The Sava is therefore the canonical answer when the question specifies the Danube’s longest right-bank tributary. It is also historically important, having been known to ancient Greeks as the Saos and to Romans as the Savus.