What valve is incorporated into the oxygen hose attachment fitting to allow breathing ambient air if the OBOGS fails?

The correct choice highlights the presence of an anti-suffocation valve in the oxygen hose attachment fitting of the T-6 aircraft. This valve is specifically designed to provide a critical safety feature by allowing the pilot to breathe ambient air in the event of an OBOGS (On-Board Oxygen Generating System) failure.

When the OBOGS system is compromised, the anti-suffocation valve automatically opens, enabling the pilot to access outside air instead of being confined to a purely oxygen-based supply. This mechanism is vital for maintaining pilot safety and ensuring continued oxygen availability during emergency situations.

In contrast, the other options relate to different functions that do not specifically address the issue of oxygen system failure and the need for ambient air access. For example, pressure relief valves serve to prevent over-pressurization in systems, ventilation control valves regulate airflow in environmental systems, and environmental control valves manage temperature and air quality, none of which have the specific function of allowing ambient air intake in a failure scenario.