Atrust is? a. single business that has total control over one industry b. economic “ups and “downs” of the stock market c. a groups of companies organized to benefit from high prices they all agreed to charge d. competitive pricing for products within a given industry

The correct option is A.
A trust refers to a large business with significant market power, that is, a company that exhibit monopoly or near monopoly and which has eliminate its competitors. Cooperate trust were initially established mainly to improve the organization of large businesses. 

The answer is c. groups of companies organized to benefit from high prices they all agreed to charge