Final answer:
Most energy in a food web is lost as heat due to metabolic processes and the second law of thermodynamics. Only about 10% of energy is transferred from one trophic level to the next, with the rest being dissipated.
Step-by-step explanation:
In a food web, most energy that is not used by organisms for growth, reproduction, and other vital processes is lost as heat to the environment. Only about 10% of the energy at one trophic level is passed on to the next level. Therefore, the majority of energy, around 90%, is actually used for metabolic activities or dissipated as heat. This is a result of the laws of thermodynamics, specifically the second law which states that some energy is always lost as heat when energy is transferred or transformed. In a food web, detritus also plays a role in the energy flow as decomposers break down dead organic matter, releasing nutrients back into the ecosystem, but these decomposers also use some of that energy and release heat. Hence, the correct answer to where most energy ends up in a food web is (c) lost as heat.