Final answer:
The First Nations group that formed Eeyou Istchee, specifically the James Bay Cree, did so to represent their rights to self-determination and economic development in Canada.
Step-by-step explanation:
The Indigenous group that developed a modern governing body called Eeyou Istchee to represent their rights to self-determination and economic development is part of the First Nations in Canada. The Eeyou Istchee, also known as the James Bay Cree, established a form of self-government to maintain and advance their cultural preservation, land rights, and autonomy following the historical context of the Canadian government acknowledging the inherent right to self-government for Indigenous groups. As a result of the historical treaties and the Indian Act of 1876 that imposed controls over the Indigenous Peoples, there has been a push towards recognizing Indigenous rights and self-governance. This move towards self-governance, such as the creation of Eeyou Istchee, is pivotal in granting Indigenous groups more control over their lands and destinies, as well as contributing to their economic development and political participation within Canada.