Assembly by Mongol chieftains was known as:

Correct option is C

The assembly by Mongol chieftains was known as Quriltai.
Quriltai was a political and military council of ancient Mongol and Turkic chiefs and khans. These assemblies were convened to make major decisions, such as electing a new Great Khan (Qa'an), planning military campaigns, or proclaiming new laws.

Additional Knowledge

(1) Ulus: A term meaning 'country,' 'district,' 'people,' or 'state,' often referring to the territorial appanages (divisions) of the Mongol Empire assigned to the sons of Genghis Khan.

(2) Tama: A military force or garrison sent to occupy and administer conquered territories, particularly in the later Ilkhanate and Golden Horde periods.

(4) Qa'an: The title for the Great Khan or Emperor of the Mongol Empire, superior to the title 'Khan'. It was bestowed upon the ruler elected by the Quriltai.