Among European countries, Ukraine shares the longest land border with Russia.
The Russia–Ukraine frontier is roughly 1,900 kilometres long, running from the northern borderlands toward the Sea of Azov and the Black Sea region. It crosses forests, farmland, river valleys and the broad steppe of eastern Europe, making it one of the continent’s most consequential borders.
A common mix-up is Kazakhstan. Kazakhstan shares Russia’s longest international land border overall—about 7,600 kilometres—but Kazakhstan is generally classified as a transcontinental country whose territory is primarily in Central Asia. When the question is limited to European countries, Ukraine is the canonical answer.
Border measurements can vary depending on whether disputed or occupied territories are included and how rivers and enclaves are calculated. Those technical differences do not change the quiz answer among the listed European options: Ukraine’s frontier is longer than Russia’s borders with Finland, Belarus or Poland.