The correct answer is C) a natural fiber tag line, saddle and hose. When using a portable tank cleaning machine, the weight of the machine is typically suspended by a natural fiber tag line, a saddle, and the hose. This setup allows the machine to be easily maneuvered and positioned within the tank during the cleaning process. The natural fiber tag line provides a secure and flexible means of suspending the machine, while the saddle helps distribute the weight evenly. The hose also contributes to supporting the machine's weight and positioning. The other options are incorrect because: A) solid metal bars clamped to the Butterworth opening would not provide the necessary mobility and flexibility required for tank cleaning; B) a portable davit is not the typical method used for suspending portable tank cleaning machines; and D) a wire rope suspension line would not be as suitable as a natural fiber tag line for this application.