Answer: The correct answer for the fill in the blank is B) Through the process of photosynthesis.
Solar energy is the ultimate source of energy for all living organisms. Most of the energy enters the ecosystem by a process called as photosynthesis.
Photosynthesis is a process by which green plants ( autotrophs) prepare their food, which is then used by other living organisms ( heterotrophs). In this process, solar energy is absorbed by the green pigment chlorophyll ( present in the chloroplast of green plants) in the presence of CO₂ and H₂O to form organic substance, which is glucose.
Thus, through the process of photosynthesis, most of the energy enters the ecosystem.