The correct answer is "conquering Siberia and parts of Central Asia."
Ivan III "the Great" expanded his rule of the principality of Moscow by freeing and uniting the surrounding Russian principalities under the control of the Golden Horde. Also, he was keen on the Byzantine culture and even married a princess of the royal house of Constantinople, Sophia (Zoe) Paleologue. This prompted Ivan's spokesman to refer to Moscow as "the Third Rome."
The conquest of Central Asia territories and Siberia would be carried out by his grandson, Ivan IV "The Terrible."