Questions: What gas was missing from the Earth's atmosphere during the Archean? (A) ammonia (B) helium (C) oxygen (D) carbon dioxide

The answer is (C): oxygen.

Explanation for each option:

(A) Ammonia: Ammonia was present in the early Earth's atmosphere, primarily as a result of volcanic outgassing and other chemical processes.

(B) Helium: Helium is a noble gas that is present in the Earth's atmosphere in trace amounts. It is not a significant component of the atmosphere and was not "missing" during the Archean.

(C) Oxygen: Oxygen was largely absent from the Earth's atmosphere during the Archean eon. It was not until the Great Oxidation Event, which occurred around 2.4 billion years ago, that significant amounts of oxygen began to accumulate in the atmosphere due to photosynthetic organisms.

(D) Carbon dioxide: Carbon dioxide was present in the Earth's atmosphere during the Archean. It was a major component of the atmosphere, contributing to the greenhouse effect that kept the planet warm despite the faint young Sun.

In summary, the gas that was missing from the Earth's atmosphere during the Archean was oxygen.