Which of the following types of equipment should be used as a source of breathing air ONLY in emergency situations?

The oxygen breathing apparatus is designed specifically for use in emergency situations, particularly where the atmosphere is unsafe due to contamination or a lack of oxygen. This type of equipment provides a supply of pure oxygen and is typically used in scenarios involving fire fighting, rescue operations, or environments where the air quality is compromised.

While other options like the self-contained breathing apparatus are also intended for emergency use, they are generally employed in various situations including active firefighting and hazardous material handling, not exclusively in emergencies. The supplied-air respirator is used in scenarios where clean air can be supplied from a distant source but still requires a safe environment to operate effectively.

Thus, the oxygen breathing apparatus is clearly designated for emergency situations due to its specific design to supply oxygen quickly and efficiently when needed the most, making it the correct choice.