Which theory believes that groups in society are engaged in a continuous power struggle for control of scarce resources?

The answer is Conflict Theory. Sociologists believe in a micro theory, based on small scale interactions, one-to-one interactions. The large scale or macro interactions are Conflict Theory which is included in the question, fundamentally people are struggling with and against one another for scarce resources. The other macro interaction theory is the theory of Fuctionalism, which focuses on how working together and/or in coordination is beneficial to the society in large and how Functionalism helps provides stability for society.