Till today, the most important in genetic engineering of plants has been the Ti plasmid of soil bacterium, Agrobacterium tumefaciens. E.Coli has been extensively used as "work horse" for genetic engineering e.g., production of humulin, somatotropin. E. coli is one of the most thoroughly studied of all living things. It is a favorite organism for genetic engineering as cultures of it can be made to produce unlimited quantities of the product of an introduced gene. Several important drugs (insulin, for example) are now manufactured in E. coli. Thus, option B is correct. Nitrosomonas, Klebsiella, Nitrobacter, Azotobacter, Rhizobium and Diplococcus are not used in genetic engineering.