When one firm in the breakfast cereal market started an advertising campaign that stressed the nutritional value of its cereals, all other cereal manufacturers started similar advertising campaigns. this suggests that the breakfast cereal market

Monopolistic

Explanation:

A market where only one company is responsible for offering products and services to the consumers in the market then such a market is known as the monopolistic market. In a monopolistic market, the control lies in the hands of a single company that performs actions such as setting the costs and fixing the supply of the product or service. In the given question, the advertising campaign has been started by one firm and then followed by other manufacturers and thus, it suggests that it is an example of a monopolistic market.