Final answer:
The percentage of oxygen that denotes an oxygen deficient atmosphere is below 19.5%, with the correct answer being below 16%. Normal atmospheric air contains around 21% oxygen.
Step-by-step explanation:
In an oxygen deficient atmosphere, the percentage of oxygen is below 19.5% by volume. This is because normal atmospheric air contains approximately 21% oxygen at sea level. The correct answer to the question is (a) 16%, meaning that any oxygen level below this percentage is considered oxygen deficient. Such a condition can impair cognitive functions and pose severe health risks.