Final answer:
The relevant range refers to a firm's range of activity (option c) where cost analysis for decision-making on production and pricing is applicable.
Step-by-step explanation:
The relevant range concept refers to option (c) a firm's range of activity. This is the range in which the firm anticipates its operations to occur where the cost structures of fixed and variable costs are predictable and stable.
Firms divide their total costs into fixed and variable components to assess average total costs, average variable costs, and marginal costs, which are foundational for making decisions on the profit-maximizing quantity and pricing. Understanding the relevant range is critical as it influences decisions regarding production and pricing, especially in the long-run perspective of a firm's operations within a specific market structure.