Biogeochemical cycles are crucial to ecosystem function because

The correct option is B.

Explanation:

The options attached to the question are given below:

A. They keep the planet warm enough for living things to survive

B. Nutrients and other life-sustaining molecules are in limited supply and must be continually recycled

C. Energy flows through ecosystems in one direction only and is eventually dissipated as heat

D. They remove poisons and keep them locked up in "sinks"

 Biogeochemical cycles refers to the pathways by which chemical substances move through the abiotic and biotic segments of the earth. The most important biogeochemical cycles include: carbon, nitrogen, phosphorus, oxygen and water cycles. Biogeochemical cycles transport and store important substances so that they can be available for the use of living organisms.  

Biogeochemical cycles are very crucial to living organisms because they help to recycle nutrients and other life sustaining molecules, which are in limited supply and which must be continuously recycled.