Why does trash decompose so slowly in landfills? The design of the landfill isolates the trash from decomposing. The types of trash that are found in landfills keep it from decomposing. The location of the landfill prevents the trash from decomposing. The presence of landfill gas prevents the trash from decomposing.

The reason for the trash decomposing so slowly is that the design of the landfill does not allow for the oxygen to pass though the landfill, and therefore the decomposition or breaking down of the trash is not allowed to happen. So the correct answer is:

The design of the landfill isolates the trash from decomposing.

(although I would find the verb "prevents" better here)