Which of the following cylinders requires the Pin-index Safety System for its attachments?

The Pin-Index Safety System (PISS) is specifically designed for small size gas cylinders, which are typically used in portable applications, such as oxygen tanks for home care or emergency transport. This system consists of specific pin arrangements that prevent the accidental connection of a regulator to the wrong type of gas cylinder. Each type of gas has a unique pin placement, ensuring that equipment is connected only to compatible gas sources, which enhances safety during respiratory therapy.

Large size cylinders generally use the American Standard Safety System (ASSS) with threaded connections rather than the PISS, making it inappropriate to apply the PISS to these larger cylinders. Therefore, the distinctive design of the PISS and its intended purpose for small size cylinders correctly identifies this as the appropriate answer.