The correct answer is - Mongolia.
Out of the suggested countries, Mongolia is the one that has by far the greatest longitudinal extent. Mongolia is a country in East Asia, located between the two giants, Russia on the north, and China on the south. The country has a relatively large longitudinal extent, while the latitudinal extent is smaller. It is the 19th largest country in the world.
The geography of Mongolia differs from place to place, with large ares being arid and semi arid plateaus, low grassland plains, high mountains, and dense taiga forest in its north.