Final answer:
An atmosphere with less than 19.5% oxygen by volume is known as an oxygen-deficient atmosphere, which is option a). Early Earth's atmosphere had low oxygen levels until the evolution of photosynthetic organisms.
Step-by-step explanation:
An atmosphere containing less than 19.5% oxygen by volume is referred to as an oxygen-deficient atmosphere. This means that the correct answer to the student's question is a) Oxygen-deficient atmosphere. Oxygen levels in the atmosphere are critical for various processes, including human respiration and combustion. An oxygen-deficient atmosphere can be hazardous since it may impair cognitive function and can lead to asphyxiation. Conversely, an oxygen-enriched atmosphere (greater than approximately 21% oxygen by volume) increases the risk of fire or explosion.
In the context of Earth's history, the primitive atmosphere contained much less molecular oxygen. Oxygen is a byproduct of photosynthesis, which implies there was very little oxygen until photosynthetic organisms evolved. Early Earth had a reducing atmosphere rich in methane and ammonia, which shifted to an oxidizing atmosphere as life forms that could perform photosynthesis emerged and produced oxygen.