Final answer:
Scientists analyze historical atmospheric CO2 levels using ice cores, which contain trapped air bubbles that offer direct evidence of past atmospheric conditions, including greenhouse gases.
Step-by-step explanation:
The option that most helps scientists analyze the amount of CO2 present thousands of years ago is C) Ice cores. Ice cores are obtained by drilling into ice sheets or high mountain glaciers and contain trapped air bubbles from the past, which provide scientists with direct samples of the Earth's atmosphere throughout history.
These ice cores show atmospheric concentrations of greenhouse gases and help researchers understand past climate conditions. For example, Antarctic ice cores have revealed fluctuations in the Earth’s temperature and atmospheric CO2 levels over the past 400,000 years, illuminating periods of natural climate cycles and the significant rise in CO2 levels since the Industrial Revolution.