The correct option is A a-4, b-3, c-2, d-1
Ernst Mayr proposed the biological species concept based on Darwin's theory of natural selection. Hence he is considered to be the Darwin of 20th century.
R.H. Whittaker proposed the Five Kingdom Classification.
Beijerinck described the viral extract obtained from tobacco plant infected with tobacco mosaic virus as contagium vivum fluidum meaning living infectious fluid.
Diener discovered viroids, one of the most infectious particles made up of ssRNA.