Question

according to some scientists, which is a cause of global warming?

increase in carbon dioxide
decrease of methane gas
increase in cloud cover
decrease of nitrous oxide

Explanation:

Global warming is caused by greenhouse gases. Carbon dioxide ($CO_2$) is a major greenhouse gas, and an increase in its concentration traps more heat. A decrease in nitrous oxide or methane, or an increase in cloud cover (which can reflect sunlight) would not cause global warming. So the increase in carbon dioxide is a cause.

Answer:

O increase in carbon dioxide