Final answer:
A firm has a sustainable competitive advantage when it implements a strategy that creates superior value for customers and competitors are unable to duplicate or find too costly to try to imitate.
Step-by-step explanation:
A firm has a Sustainable competitive advantage when it implements a strategy that creates superior value for customers and competitors are unable to duplicate or find too costly to try to imitate. This means that the firm has a long-term advantage over its competitors, allowing it to earn above-normal profits consistently. An example of a sustainable competitive advantage is Apple's strong brand reputation and loyal customer base, which competitors find difficult to replicate.