In the context of sustainable competitive advantage, unlike rare resources, nonsubstitutable resources cannot be

a. replaced by other resources to produce similar value

Explanation:

Options are "a. replaced by other resources to produce similar value. b. used by firms to improve their effectiveness.  c. used to sustain a competitive advantage.  d. controlled or possessed by many competing firms."

In the context of sustainable competitive advantage, unlike rare resources, non substitutable resources cannot be replaced by other resources to produce similar value. Rare and valuable resources can produce sustainable competitive advantage only if they are non-substitutable resources. In other words, sustainable competitive advantage means when no other resources can replace and produce similar value or competitive advantage.