maintain that in capitalistic societies, where a few people control the labor of many, the social structure reflects a system of relationships of domination and subordination.

The answer is conflict theorists.

Explanation:

Conflict theories refer to some perspectives that belong to sociology and social psychology in which there is a materialist interpretation of history,  dialectical method of analysis, as well as a political program of the revolution.  

Conflict theories are also related to power differentials like class conflict. Karl   Marx is considered the person who established social conflict theory, which is also considered an important component related to the four major paradigms of sociology. Some conflict theories try to expose the different ideological aspects concerning traditional thought.      

conflict theorists.

Explanation:

In order to explain capitalism, Karl Marx has to go further to explain what conflict theory is. Just like in agricultural society there is inequality between people who owns what is been used in production and those that work for the production. conflict theory is all about competition. So, when we have a limited resources and the people compete for this resources the theory backing it up is the conflict theory.

(Note the capitalized words are the missing words used in filling the gap) Like it is given in the question;

"CONFLICT THEORISTS maintain that in capitalistic societies, where a few people control the labor of many, the social structure reflects a system of relationships of domination and subordination."