In a compost pile, food and dry leaves are broken down into simpler substances by bacteria and fungi. what carbon cycle process is at work?

ammonification
decomposition
photosynthesis
respiration

 Decomposition

Decomposition is a process, during which microbes like bacteria and fungi gets colonize over the dead remains of plants and animals. A compost pile is created when the partially degraded material obtain from plant and animals gets mixed with the components of soil. The compost pile allows the colonization of microbes. These microbes convert the contents of the compost pile into simpler substances or elemental forms. The organic matter present in the compost pile is a rich source of carbon. The decomposers feed on the nutrients present in the compost pile. They release carbon to the soil and the atmosphere in the form of carbon dioxide by the process of respiration.  

Therefore, in the given condition, the carbon cycle process is at work is decomposition.

answer;

c. a tree whose roots are surrounded by wet sand.

explanation;

water stress is an important factor limiting the growth and productivity of forests.water stress is the most often cited cause of poor survival and growth of woody landscape plants.

-insufficient soil moisture is the most common form of water stress, but excess moisture that leads to poor soil aeration (hypoxia) can also result in reduced plant vitality. a tree whose roots are surrounded by wet sand would be least likely to experience water stress.