Question

Which of these thinkers worked with Karl Marx to develop the theory of communism?

Answer 1

It was "Friedrich Engels" who worked with Karl Marx to develop the theory of communism, since both of these men were ultimately interested in finding ways to reduce economic and social inequality. 

Answer 2

Friedrich Engels worked with Karl Marx to develop the theory of communism.

Friedrich Engels  was a German philosopher, communist, social scientist, journalist and businessman.

Engels developed the Marxist theory along with Karl Marx and in 1845 he published The Condition of the Working Class in England, based on personal observations and research in English cities. In 1848, he co-authored The Communist Manifesto together with Marx and authored and co-authored with Marx many other works as well.