Which of the following is not a possible effect of increasing carbon dioxide levels in the atmosphere?

a. increase in water contamination and floods

b. ocean acidification

c. cutting down forests

d. reduction of cold-related deaths

The question is asking us "Which of the following is not a possible effect of increasing carbon dioxide levels in the atmosphere?" If you look at option 3, c. cutting down forests, you can see that this is a human activity - and a human activity is caused by human decisions and only indirectly by the level of Carbon dioxide. Therefore, the best answer is c. Cutting down trees

Out of the following choices, the answer which is not a possible effect of increasing carbon dioxide levels in the atmosphere is cutting down forests. So the answer is. D