What must each gauge and instrument in the fire apparatus be marked with?

Each gauge and instrument in the fire apparatus must be marked with a function label to clearly indicate its purpose and the information it provides. This labeling is critical for operators to quickly understand the readings and functions without confusion, especially during emergency situations when time is of the essence. Function labels ensure that all personnel operating the apparatus can effectively and safely interact with the equipment, as they provide essential information on what each gauge or instrument measures.

In the context of the other options, marking gauges with a serial number, psychological history review, or manufacturer's warranty does not directly contribute to the operational clarity needed in emergency responses. A serial number is primarily for identification and tracking purposes, while a psychological history is not relevant to the operation of mechanical devices. A manufacturer's warranty speaks to product guarantees and does not aid in the immediate use or understanding of the gauges during operation. Thus, the function label is the most pertinent requirement for safety and efficiency.