Final answer:
The primary factors behind the increase in atmospheric CO₂ levels are human activities, notably the burning of fossil fuels and deforestation. Natural processes like volcanic eruptions have a minor role in comparison.
Step-by-step explanation:
The increase in atmospheric CO₂ can be attributed to several human activities. The primary mechanism responsible for this rise is the burning of fossil fuels, such as coal, oil, and natural gas, which releases significant amounts of CO₂ into the atmosphere. Other significant activities include deforestation, which reduces the number of trees that can absorb CO₂ through photosynthesis, and various industrial processes like cement manufacture. Agriculture, particularly animal husbandry, also contributes to increased CO₂ levels through respiration and methane production.
While natural processes such as volcanic eruptions and animal respiration do release CO₂, their contribution to the recent drastic increase is relatively minor compared to human activities. Therefore, the correct answer to the question is a) Deforestation and volcanic eruptions, with a strong emphasis on human activities, particularly the burning of fossil fuels and deforestation, being the dominant factors.