We use antisera to distinguish between various within a species of bacteria.
isotypes
serovars
subspecies
lines

Serovars

Explanation:

Serovars refer to the different strains of a species of microbe that differ from each other with respect to the antigenic composition of a specific structure such as polysaccharides of the cell wall. Serovars are identified suing appropriate antiserum.  

For example, the bacterial species Streptococcus pneumoniae have as many as 90 serovars that differ from each other with respect to antigens present in their capsules. These different serovars are identified by using antiserum for specific capsular types which in turn would induce capsular swelling.  

the answer would be..

b. sucking