Which of the following factors is most controllable in a fire pump setup?

The most controllable factor in a fire pump setup is the diameter of the hose. Firefighters and operators can select the appropriate diameter for hoses based on the specific needs of the operation and the conditions they are facing. This selection can significantly impact both flow rate and pressure, allowing the operator to effectively manage resources during firefighting operations.

In contrast, other factors like elevation difference, pump capacity, and ambient conditions can be much less controllable. Elevation difference is often a fixed condition determined by the topography of the area where the fire is being fought; operators must account for this when calculating the pressure needed at various heights. Pump capacity is typically set by the equipment specifications, meaning that while it can be operated at its rated capacity, it cannot be changed on the fly or adjusted in a typical emergency situation. Lastly, ambient conditions such as temperature and weather are outside the control of those operating the fire pump and can only be responded to, rather than adjusted. Thus, the hose diameter is the most adaptable variable in this scenario.