Which of the following is an autotrophic bacterium?

Autotrophic bacterium is an organism that produces complex organic compounds from simple substances using light or inorganic chemical substances as the source of energy. Bacteria using light as energy source are  called as photoautotrophs. Bacteria using inorganic substances as energy source are called as chemoautotrophs. Nitrifying bacteria like Nitrosomonas and Nitrobacter are examples of autotrophic bacteria.

Thus, the correct answer is 'Nitrobacter.'