Final answer:
An oxygen-deficient atmosphere is recognized when the oxygen concentration is below 19.5% by volume. The correct answer from the given options, indicating an oxygen-deficient environment, would be 'a) 16%'. Normal atmospheric oxygen levels are around 20.9%.
Step-by-step explanation:
In an oxygen-deficient atmosphere, the percentage of oxygen is below 19.5% by volume. Normal atmospheric air contains approximately 20.9% oxygen, and oxygen levels below this can impair vital physiological processes. For safety reasons, occupational guidelines often consider environments with less than 19.5% oxygen as oxygen-deficient. Breathing air with a low partial pressure of oxygen can lead to decreased oxygen transport in the blood, affecting tissue oxygenation and potentially causing symptoms such as impaired coordination, judgment, and at more severe levels, unconsciousness or even fatality.
The correct choice for the given options would be 'a) 16%' since it is below the forementioned threshold of 19.5% that marks oxygen-deficient conditions. However, the answer choice 'c) 20.9%' reflects the typical, healthy concentration of oxygen in Earth's atmosphere.