Answer:
The correct answer is letter "C": a sustainable competitive advantage.
Explanation:
A competitive advantage is an advantage a firm has over its competitors. Essentially, a competitive advantage is what enables a firm to earn profits through greater sales or margins. There are two main types of competitive advantages: comparative advantage refers to a firm's ability to produce a good or service at a lower cost than competitors and; differential advantage when a firm offers goods or services different to its competitors due to a unique feature. The more sustainable is a firm's competitive advantage, the more difficult for competitors to neutralize it.
In the case, Amazon broke Barnes and Noble position in the book retailing market easily without giving an option for Barnes and Noble to recover. This implies Barnes and Noble's competitive advantage was not sustainable.