Final answer:
Mongolia was part of China in 1900,
Therefore, the correct answer is: option 2) China
Step-by-step explanation:
In 1900, Mongolia was a part of the Qing Empire, which indicates that it was under the rule of China at that time.
Mongolia did not become part of the Soviet Union until after the 1917 Bolshevik Revolution when it received support from the Soviets and became a communist state.
During the Cold War era, Mongolia was used by the Soviet Union as a buffer state against China. However, it's important to note that the modern nation-state of Mongolia did not exist as an independent entity in 1900; it was an integral part of the Chinese territory.
Mongolia's significant history also includes the establishment of the Mongol Empire by Genghis Khan in the 13th century, and today, its capital Ulan Bator is a major city representing about one-third of its population.