The correct option is B A fungus
The discovery of each of the five major groups of plant growth regulators (PGRs) have been accidental. The bakane (foolish seedling) a disease of rice seedlings, was caused by a fungal pathogen Gibberalla fujikuroi. Japanese scientist, E. Kurosawa reported the appearance of symptoms of the disease in uninfected rice seedlings when they were treated with sterile filtrates of the fungus. The active substances in the extract were later identified as gibberellic acid.