Hi, did you forget to mention that this question is about the article "The Rise of Oxygen in the Earth's Atmosphere" by American Museum of Natural History
Answer:
D. when oxygen increased in Earth’s early atmosphere
Step-by-step explanation:
Right at the beginning of the article we can find out what scientists are trying to discover. This is because the introduction of the article introduces us to Grant Young and Jay Kaufman, a university professor and a geologist, respectively, who are the main scientists in the study that seeks to discover and set firm dates for the increase in oxygen in the Earth's primitive atmosphere.