The theory of comparative advantage:

a-claims that economic well-being is enhanced if each country's citizens produce only a single product.

b-claims that economic well-being is enhanced when all countries compare commodity prices after adjusting for exchange rate differences in order to standardize the prices charged all countries.

c-claims that economic well-being is enhanced if each country's citizens produce that which they have a comparative advantage in producing relative to the citizens of other countries, and then trade production.

d-claims that no country has an absolute advantage over another country in the production of any good or service.