Ecosystems sustain life by regulating the flow of energy and recycling nutrients through biogeochemical cycles such as the carbon, nitrogen, and water cycles.
These processes are essential for maintaining life on Earth.
Hence,
\[
\boxed{\text{They maintain biogeochemical cycles and energy flow essential for life.}}
\]
Therefore,
\[
\boxed{(C)}
\]
is the correct answer.