karl marx developed the economic theory of\no socialism.\no capitalism.\no utopianism.\no communism.

Brief Explanations:

Karl Marx is a key figure in developing the theory of communism. His works like Das Kapital analyze the capitalist - mode of production and lay the theoretical foundation for communism. Socialism has earlier roots and different strands before Marx, capitalism was already an existing economic system studied by many before Marx, and utopianism was criticized by Marx as lacking a scientific basis for social change.

Answer:

communism.