The right answer is A) Uses H2S for an energy source.
Beggiatoa is a bacterium that lives in sulphide-rich environments. They use H2S as a source of energy.
Purple sulfur bacteria, are also bacteria that depend on the presence of H2S but do not use it for the purpose of producing energy. They use it as the ultimate electron acceptor in its respiratory chain (instead of oxygen), since they are anaerobic bacteria.