Realpolitik is a 19th   century German term, indicating the adoption of limited objectives that have a fair chance of success. The correct answer is A. The definitons for the terms taking place in the other options as follows:

Power is defined as the capability to induce or force another to comply with one’s wishes: either to start or continue an activity, or to cease that behavior.

Neorealism is also called “structural realism” because it emphasizes the design and structure of the world system rather than stressing man’s natüre and the states as units, as realism does.

Marxism is a school of thought inspired by the writings of Karl Marx, and refined by V. I. Lenin and others.   

Epistemology: the study ofmknowledge and justified belief. As the study of knowledge, epistemology is concerned with the following questions: What are the necessary and sufficient conditions of knowledge? What are its sources? What is its structure, and what are its limits? How is knowledge justified? Epistemology addresses issues having to do with the creation and dissemination of knowledge in areas of inquiry. It examines “how we know what we think we know”