Final answer:
The long-run decision is defined as the ability of a firm to change its scale of operation, adjust all factors of production, and make strategic decisions such as building new factories or purchasing new machinery. This distinguishes the long run from the short run where firms cannot change their usage of fixed inputs.
Step-by-step explanation:
The long run is the period of time when all costs are variable. In the long run, a firm can adjust all factors of production, including changing its scale of operation, building new factories, purchasing new machinery, or closing existing facilities. Therefore, option a. The firm can change its scale of operation defines a long-run decision. Option b. The firm can exit the business is not necessarily a characteristic of the long-run decision, as a firm can exit the business in both the short run and the long run. Option d. The firm has no fixed costs is also not true, as a firm may still have some fixed costs in the long run. Therefore, the correct answer is option a.