Which one of the following respirators provides clean air from a tank?

The correct choice is a Self-Contained Breathing Apparatus (SCBA), which provides clean air from a tank. SCBAs are designed to protect the user in environments where the air may be contaminated or lacking in oxygen. They consist of a high-pressure tank filled with breathable air, a harness system to secure the tank, and a mask that covers the face to deliver clean air to the user.

In contrast, other types of respirators, such as half mask respirators and air-purifying respirators, do not provide fresh air from a tank. Instead, half mask respirators cover only the nose and mouth and require the presence of clean air in the environment, while air-purifying respirators filter out harmful particulates or gases from the surrounding air but do not supply clean air independently. The Supplied Air Respirator, while it does provide a supply of clean air, typically relies on external air sources and pipes, rather than an independent tank. Thus, the SCBA is uniquely suited for situations requiring a portable and self-sufficient source of clean air.