In South Bihar, the locals have a common resentment against migrant traders and moneylenders. What are these migrant groups known as?

The correct option is Dikus.

In the context of the history of South Bihar and Jharkhand (historically part of the Bengal Presidency and later Bihar), the term Dikus was commonly used by the local tribal communities (such as the Santhals, Mundas, and Oraons) to refer to outsiders, foreigners, or migrants.
These outsiders included migrant traders, moneylenders, and British officials who entered their land, grabbed their territories, and pushed the local inhabitants into debt traps and bondage.

Here is a breakdown of why this term is correct, along with context on the other options:
1. Dikus: This term literally means "outsiders" or "foreigners" in tribal languages of the region. The tribal populations resented them because the moneylenders charged exorbitant interest rates and the traders sold goods at high prices, leading to severe economic exploitation.
2. Nakarattars: This refers to a prominent merchant/banking community (specifically Nattukottai Chettiars) from Tamil Nadu in South India, not South Bihar.
3. Adivasis: This is a collective term for the indigenous tribals of India, who were the victims of the exploitation, not the migrant traders.
4. Oraons: This is a specific tribal group inhabiting parts of Jharkhand, West Bengal, Odisha, and Bihar, rather than a term for migrant traders.