Randy wants to know whether a soil's porosity affects how easily seedlings grow in it. to figure this out, he needs to design an experiment in which he plants seedlings in soils with levels of porosity and levels of permeability. a. different; different b. different; equal c. equal; equal d. equal; different

Since the main objective of this experiment is to determine the effect of porosity on seedling growth that should be the only independent variable. In short, that is the only variable that should be different to ensure fair testing. 

The answer should be B:

he plants seedlings in soils with different levels of porosity and equal levels of permeability. 

Permeability is not what needs to be tested. If it changes, you may not be able to determine whether it was the porosity or permeability that cause changes.