A group of firms manufactures writing implements such as pens, pencils, and markers. This group should be referred to as a (n) : a. service provider. b. substitute. c. market segment. d. regulator. e. industry.

E. Industry

Explanation:

Industry or an industry is a group of firms/businesses/manufacturers a particular kind of goods and services. In this case, the firms that makes up the industry all manufactures writing implements (such as pen, pencils and markers). Simply put, an Industry is a sector that produces good or related services within an economy.

When classifying a firm into an industry, the major source of revenue is the necessary indicator used in doing so.