Mahajanapadas were ancient Indian kingdoms that emerged around the 6th century BCE. They were characterized by their territorial expansion, economic prosperity, and political organization. There were sixteen Mahajanapadas in total, and each had its own distinct characteristics and ruling dynasties.

The options given in the question are:

1. Vajji
2. Malla
3. Vatsa
4. Assaka

The correct answer is option D, which means none of the above is not a Mahajanapada. Let's discuss each option in detail:

1. Vajji:
Vajji was indeed one of the Mahajanapadas. It was located in the northern part of ancient India, in present-day Bihar and Uttar Pradesh. The Vajji Mahajanapada was ruled by a confederation known as the Vajjian Republic. It was known for its strong political and administrative system and was considered one of the most prosperous Mahajanapadas.

2. Malla:
Malla was also one of the Mahajanapadas. It was located in the region of present-day Uttar Pradesh and Madhya Pradesh. The Malla Mahajanapada was divided into two major kingdoms - the Eastern Malla and the Western Malla. It was known for its skilled warriors and played a significant role in the political landscape of ancient India.

3. Vatsa:
Vatsa was another Mahajanapada. It was situated in the present-day state of Uttar Pradesh. The Vatsa Mahajanapada was ruled by the famous king Udayana, who was known for his diplomacy and political alliances. Vatsa was a prosperous kingdom with a well-developed economy.

4. Assaka:
Assaka, also known as Ashmaka, was indeed one of the Mahajanapadas. It was located in the Deccan region, in present-day Maharashtra. The Assaka Mahajanapada was ruled by various dynasties and had close trade connections with other Mahajanapadas.

Therefore, all the options given in the question - Vajji, Malla, Vatsa, and Assaka - were Mahajanapadas. None of them is not a Mahajanapada, so the correct answer is option D.