We can sort the firms based on the source of their market power, either limited competitors, product differentiation, both, neither as follows;
ADM - Limited Competitors
Heartland Mills - Limited COmpetitors
McDonald's - Product differentiation
Toyota - Product differentiation
Limited Competition implies that the market is not saturated and the company can afford to have some control over the price of their products. This is the case for the companies, ADM and Heartland Mills.
Product differentiation can be seen in McDonald's and Toyota as they make their products unique so that they can have a high value to consumers. This strategy enables them to fix premium prices.