Final answer:
The most effective way to reduce greenhouse carbon dioxide is through afforestation, which involves planting more trees.
Step-by-step explanation:
The most effective way to reduce greenhouse carbon dioxide is option a) Afforestation, which involves planting more trees. Trees naturally absorb carbon dioxide from the atmosphere through the process of photosynthesis. By increasing the number of trees, we can effectively reduce the amount of carbon dioxide in the atmosphere. This helps to mitigate the impact of global warming and climate change.