The correct answer is nutrient cycling.
Indirect ecosystem services are benefits provided by ecosystems that do not involve direct consumption of resources but rather support processes and functions that are essential for the environment and human well-being, such as nutrient cycling.
Fossil fuels and medication are considered direct services since they involve direct extraction or use of ecosystem resources. Hiking is an activity that can be enjoyed due to the presence of ecosystems, but it doesn’t represent an indirect service itself.