Scientific socialism.
Karl Marx
Marx was socialist.
Karl Marx.
Karl Marx invented Radical Socialism.
It’s not clear that anyone ‘invented’ Socialism.
Karl Marx
No. He viewed what he called "utopian socialism" as too idealistic and impractical. He proposed scientific socialism instead, because he believed that socialism was the scientifically proven next stage of human economic development.
The answer is Karl Marx.
Socialism/Communism
Karl Marx had a view of a true socialist society which he termed scientific socialism.
KARL MARX had the view that history was inexorably trending to communism and that capitalism would improve to socialism and socialism would improve to communism.
Marx made a detailed analysis and critique of capitalism, and advocated its replacement by a classless Socialist society.