Step 1: Recall the Mahajanapadas.
In the 6th century BCE, sixteen Mahajanapadas existed. Each had its own capital city.
Step 2: Identify Ujjaini.
Ujjaini (modern Ujjain in Madhya Pradesh) was the capital of the Mahajanapada known as Avanti. Avanti was an important political, cultural, and trade hub of central India.
Step 3: Elimination.
- (A) Chedi: Its capital was Shuktimati.
- (B) Vatsa: Its capital was Kausambi.
- (D) Matsya: Its capital was Viratanagara.
- (C) Avanti: Correct, with Ujjaini as its capital.
Step 4: Conclusion.
Thus, Ujjaini was the capital of the Avanti Mahajanapada.