Which of the following is considered as the main ecological service?

Ecosystem services are the direct and indirect benefits that humans and other living organisms obtain from natural ecosystems.

While ecosystems provide numerous vital services such as waste decomposition, purification of water and air, and genetic resources, the most fundamental and main ecological service is the fixation of atmospheric $CO_2$ and the release of $O_2$.

This process, driven by photosynthesis in primary producers (plants, algae, and some bacteria), forms the basis of the entire food web by converting solar energy into usable chemical energy (organic matter).

Furthermore, the continuous release of oxygen is essential for the survival of almost all aerobic life forms on Earth, making it the most critical and primary service upon which other biological processes depend.

Answer: Fixation of atmospheric $\text{CO}_2$ and release of $\text{O}_2$