Answer:
Increase the sunlight
Step-by-step explanation:
Photosynthesis is the most important phenomenon for increasing the biomass of plants. This involves the conversion of light energy into chemical energy. The main components needed to carry out this process are CO₂, sunlight and water. The plants and trees are the autotrophs because they can synthesize food for themselves as well as for the consumers, by taking in CO₂, sunlight and water, and in return, it liberates food and oxygen (O₂).
So, in order to attain the increasing plant biomass by increasing the amount of CO₂ level, the amount of sunlight must be increased.