Antigenic variation is/are seen in-

Ans. is 'b' i.e., Neisseria Antigenic variation It is the process by which an infectious oganism alters its surface proteins in order to evade a host immune response. . Antigenic variation not only enables immune evasion but also allows pathogens to cause reinfection as they are not recognised by the host's immune response . . Antigenic variation enables viruses to cross the species barrier,(eg. H5Nlvirus "jumping" from birds to then infect humans). . Antigenic variation in bacteria is best demonstrate by: species of the genus Neisseria ( most notably, Neisseria meningitidis and Neisseria gonorrhoeae, the gonococcus);Borrelia recurrentis; species of the genus Streptococcus and the Mycoplasma. . Antigenic variation in viruses is best demonstrated by : by HIV ( Human Immunodeficiency Virus); Influenza virus and Hepatitis virus, rhinoviruses and enteroviruses. . Antigenic variation in protozoa is best demonstrated by : species of the genus Plasmodium ( responsible for causing malaria) and the trypanosomes ( Trypanosoma cruzi which causes Chagas disease and T. bruzi gambiense and T. bruzi rhodesiense which both cause African sleeping sickness).