Merchants and traders who travel along these routes must stop at oases to replenish food and water supplies and this means that whoever controls an oasis also controls the trade along the route—making oases desirable to political, economic, and military leaders.
I would say the best answer to this question would be option C "unelected head of state." Just because monarchy is like a kingdom and the throne can be handed down to anyone who doesn't have to be elected. It isn't none of the other options because the United States has a democracy and also has a constitution, political parties, and representatives who make laws. While monarchy is ran by a king.
Hope this helps.
