Final answer:
In the given options, Tarzan's reed "snorkel" is an example of an open breathing system because it allows for direct exchange of air with the environment without recirculation.
Step-by-step explanation:
The open breathing system referred to in the question allows air to move in and out without being recirculated within the system. Of the options provided, A) Tarzan's reed "snorkel" would be considered an open breathing system because it allows air to freely move in and out without any mechanical valves or systems to recirculate the air. The snorkel does not contain the exhaled air; it releases it into the environment. On the other hand, B) A dual limb circle circuit system, C) Bain circuit, and D) Open drop anesthesia mask are closed or semi-closed systems used within medical practice to either recirculate or control breathing in an anesthesia context.