Final answer:
An atmosphere containing less than 19.5% oxygen by volume is considered to be oxygen-deficient, which can lead to hypoxia and pose serious health risks. The option (C) is correct.
Step-by-step explanation:
An atmosphere containing less than 19.5% oxygen by volume is considered to be oxygen-deficient. Normal air has about 20.9% oxygen, and any environment with an oxygen concentration below 19.5% can be a risk to human health.
As the partial pressure of oxygen in a person's lungs is critical for life, breathing air with a lower oxygen concentration can lead to hypoxia, causing symptoms like fatigue, headaches, and confusion. Oxygen deprivation can be severe, with lower partial pressures (0.06 atm) potentially being fatal. Safety engineers ensure that environments have sufficient oxygen and monitor for oxygen-deficient hazards. Therefore, option (C) is correct.