According to the world-system theory, the world consists of
a. core nations.
c. semiperipheral nations.
b. peripheral nations.
d. all of the above

(D) All of the above

Explanation:

 The world system theory mainly consist of three level earth hierarchy theories are as follows:

The world system theory is also known as world system analysis and it is the macro scale approach that emphasize the theory of world system and the social structure in the form of primary unit. The core nation mainly focus on the high skills and the capital production.

a is the answer i believe.