Correct option is C
The chemical used for polyploidy breeding is colchicine. Colchicine is a mitotic inhibitor that prevents the formation of the spindle fibers during cell division. When applied to plants, colchicine induces polyploidy by doubling the chromosome number. Polyploidy is valuable in plant breeding as it can result in increased vigor, altered phenotypes, and enhanced traits, contributing to the development of new crop varieties with improved characteristics.