Concept:Identifying the bacterium that does not have a specific, essential role in the nitrogen cycle.
Explanation:The nitrogen cycle involves microbes that fix nitrogen, carry out nitrification, or denitrification.
Nitrosomonas oxidises ammonia to nitrites during nitrification.
Nitrobacter oxidises nitrites to nitrates during nitrification.
Rhizobium fixes atmospheric nitrogen in root nodules of legumes.
Bacillus is a large, general group of bacteria whose members are not specifically known for a major nitrogen-cycle role.
Therefore, among the listed options, bacillus is the exception.
Answer:A. bacillus