I think the answer is regiment
The correct answer to this open question is the following.
You forgot to include the options of the question, we can say the following.
The four characteristics of the Black Prarie region are the following. Several of Mississippi’s major cities are located there. Cattle and dairy farming are important to its economy. It is part of the Appalachian foothills. The plantation system was very successful there. It is home to several well-known rock and blues musicians.
The Black Praire can be found in the Northeastern part of the state of Mississippi to the central part of the state of Alabama. Its name comes from the dark and fertile soil of the region, that allow god crops for farmers. It is a place full of insects, birds, and plants.
The shared way of life is "culture" - all other options are entities, and not ways of life. This means that we can only consider options a) and d)
among those, "nation" is a better word for a political entity (but actually I would say that "country" is the best word for a political entity) - so in the end the correct answer is d)
