Which of the following is a sign of a properly functioning SCBA?

A fully charged air cylinder is a key indicator of a properly functioning SCBA. This ensures that the user has access to an adequate supply of breathable air when operating in environments that are immediately dangerous to life and health (IDLH). An air cylinder that is charged to the proper pressure guarantees not only safety but also confidence in the equipment's performance during firefighting or rescue operations.

The other options point to various deficiencies. Non-responsive alarms would indicate a malfunction in the SCBA's alert systems, which are vital for notifying the user of low air supply or other critical conditions. A loose facepiece could compromise the seal, allowing contaminants to enter, which defeats the purpose of using SCBA in hazardous conditions. Worn-out straps could affect the fit and stability of the SCBA, leading to discomfort and potential safety risks during use. Each of these aspects highlights the importance of regular equipment checks to ensure reliability and safety during operations.