The correct option is C Synthetic capsular polysaccharide material
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 infleunza and for meningitis caused by Neisseria meningitids. It is not used for the plague. To treat plague heat-killed virulent bacteria i.e., heat killed Y. pestis.
Thus, the correct answer is option C.