How do you turn on the SCBA pack?

To turn on the SCBA pack, opening the cylinder is the correct action because it allows the compressed air to flow into the breathing apparatus. When the cylinder valve is opened, it pressurizes the system and enables the user to breathe from the air supply provided. This is a crucial first step to ensure that the SCBA is operational and ready for use.

The other choices involve actions that may be part of the SCBA operation but do not specifically relate to turning the unit on. For example, pressing buttons may control alarms or indicators but opening the cylinder is the essential step that activates the breathing air system. Connecting the battery, while necessary for certain electronic components of the SCBA, does not directly turn on the unit regarding its primary function of supplying breathing air.