Which one of the following can not be used for preparation of vaccines against plague?

For vaccines, avirulent, live bacteria can be used or heat-killed suspension of virulent bacteria or formalin-inactivated suspension of virulent bacteria. However, synthetic capsular polysaccharide vaccines are there for the treatment of pneumonia caused by Streptococcus pneumoniae, Hemophilus influenzae and for meningitis caused by Neisseria meningitidis. It is not used for the plague. To treat plague heat-killed virulent bacteria i.e., heat-killed Y. pestis are used. Thus, the correct answer is option B.