The correct option is A Global warming
The release of harmful gases like carbon monoxide into the atmosphere leads to a rise in the temperature of the Earth. This gradual rise in the Earth's temperature is known as global warming. It can lead to the melting of polar ice caps and consequently cause flood.
Afforestation is the establishment of forest whereas soil erosion refers to the removal of top-most layer of soil by factors like wind and water. Hence, soil erosion and afforestation does not result in flood.