Human blood groups A, B and O were discovered by Landsteiner in 1900, while the AB blood group was discovered by de Castello and Steini in 1902.
Bernstein discovered multiple alleles, codominant and dominant - recessive relationships in the determination of human blood groups.
Calvin Bridges contribution is among the Drosophila's polytene chromosomes.
Thomas Hunt Morgan discovered the elucidating role that the chromosomes play in heredity.
Therefore, the correct answer is option A.