The answer would be B) decomposers.
Reason: Omnivores would be out, because they do not recycle anything, so to speak. They eat both plants and animals but do not recycle minerals in any way.
Carnivores eat meat, which still doesn't cycle matter.
Producers generate glucose, while recycling matter through photosynthesis and cellular respiration, but not minerals. This would be a good distractor if you didn't know the material.
Decomposers however break down dead cellular remains of other organisms into their component parts, which then creates new soil and releases the minerals back into the environment to be used again.
Hope that helped! :-)
