What Figure Did Diego Rivera Refuse to Remove from his Mural?

And the answer is ...

c) Vladimir Lenin

A known socialist, Diego Rivera depicted several scenes of Moscow May Day and a portrait of Lenin in his original mural. Although he offered to counterbalance the images with a portrait of Lincoln, he refused to remove them, causing Nelson Rockefeller to paper over the entire mural. Although restorers attempted to save it a year later, the mural was beyond repair and was destroyed. It was replaced by a mural called "American Progress," by Josep Maria Sert.