Final answer:
A G.O. is a directive issued by high-ranking authorities like the Fire Chief or Mayor, who have the power to regulate departmental operations.
Step-by-step explanation:
A G.O. (General Order) shall be defined as a directive from the higher authorities within a hierarchical organization that regulates or determines the policies, operations, or activities of the department, its divisions, and members. In the context provided, it seems most appropriate to fill in the blanks with high-ranking positions with such authority. Considering the options provided and the hierarchical structures described in the reference materials, the correct fill-ins would be from a 'Fire Chief' or 'Mayor', as these positions typically hold the power to issue directives that govern the operations within a department or city.