Final answer:
The causative agent of fowl cholera is Pasteurella multocida, a bacterium that can cause infections in animals and humans.
Step-by-step explanation:
The causative agent of fowl cholera is Pasteurella multocida. This bacterium can be transmitted from animals to humans through bites, causing infections of the skin and deeper tissues, and is also responsible for severe pneumonia in animals such as sheep and goats. In contrast, Chlamydophila psittaci causes psittacosis, a disease found primarily in birds but that can also be transmitted to humans, and Reticuloendotheliosis virus is an avian virus. Mycoplasma gallisepticum causes respiratory diseases in birds but is not the cause of fowl cholera.