Concept:An industry is not one factory or firm, but a group of firms producing similar goods.
Explanation:Option A is incorrect because one firm cannot represent an industry.
Option B is incorrect because a single factory with many products is still one business unit.
Option D is too general and does not focus on a closely related set of products.
An industry is best defined as a collection of firms that sell closely related products.
Therefore, the most accurate description is option C.
Answer:C. a group of firms that sells a closely related set of products.