Final answer:
All options listed - theory of comparative advantage, imperfect markets theory, and product cycle theory - are mentioned as theories of international business, making option d the correct answer.
Step-by-step explanation:
The question pertains to theories of international business which are essential to understanding how and why countries engage in international trade. The theory of comparative advantage is mentioned as one of these theories. It posits that countries should produce goods in which they have a comparative advantage, meaning they can produce at a lower opportunity cost relative to other countries. Following this theory enables countries, even small economies, to fully benefit from economies of scale, and consumers to enjoy a variety of products due to international competition and trade.
In the context of this question, all of the listed options, a. theory of comparative advantage, b. imperfect markets theory, and c. product cycle theory, are mentioned in the provided reference material as theories of international business. Hence, the correct answer is d. All of the above are mentioned in the text as theories of international business.