Which of the following statement about energy and nutrients in an ecosystem is true? a. energy and nutrients flow in one direction through an ecosystem. b. energy and nutrients cycle through an ecosystem. c. energy cycles through an ecosystem, while nutrients flow in one direction. d. nutrients cycle through an ecosystem, while energy flows in one direction

D. Nutrients cycle through an ecosystem, while energy flows in one direction.  
Let's use the old conservation laws
 1. Matter may be neither created nor destroyed
 2. Energy may be neither created nor destroyed, but can be converted from one form to another. 
 With those two laws in mind, let's look at the options.
 A. Energy and nutrients flow in one direction through an ecosystem. * This can be demonstrated to be false immediately. Animals eat plants. Animals excrete feces. Feces fertilize plants and the cycle repeats. So this is a bad choice, and any choice that indicates that nutrients don't cycle is also false. 
 B. Energy and nutrients cycle through an ecosystem.  * Animals eat food and metabolize it, giving off waste heat (energy) which is lost to the ecosystem. So nope, this doesn't work either. We do lose energy, so this is a bad choice. 
 C. Energy cycles through an ecosystem, while nutrients flow in one direction.  * Look at "A" above. Nutrients cycle, therefore this is wrong and don't pick it. 
 D. Nutrients cycle through an ecosystem, while energy flows in one direction. * As demonstrated above with "A", nutrients do cycle. Let's look at the energy flow. Plants get energy from the sun and produce energy rich foodstuffs. Animals eat said food and incorporate into their own energy rich structure (and tasty meat), but do lose some of that energy to the environment. Other animals eat animals and so on and so forth. So this does describe reality and is therefore the correct choice.