Which activity would best keep the world's oxygen supply from decreasing? a. planting trees in forests that have been harmed b. changing jungles into farmland c. cutting down forests to sell wood d. clearing trees from areas near rivers

a.Planting trees in forests that have been harmed 

Almost half of the world's oxygen was supplied by phytoplanktons thru the process of photosynthesis.

The other half is produced on land by trees, and other plants via sunlight (photosynthesis) also.  To stop the world’s oxygen supply from decreasing, we must plant trees that have been harmed. 

a physical change is being made.