Firms that provide components or finished goods and services to the primary firm are called . second-tier customers b. first-tier suppliers c. second-tier suppliers d. first-tier customers

B. First tier suppliers

Explanation:

First tier suppliers refers to the individual or a group who provide the main components that needed to make a certain product directly to the producer (without any intermediary)

For example In order to produce a car, you need several main components :  as the engine, the wheels. the light system, etc. All of these are the main components that couldn't be eliminate from the production.  These parts often ordered from smaller independent companies that focused its operation only to make one single parts. These independent companies are what considered to be first tier suppliers.