Concept:Rabbits digest fibrous roughages with the help of microbes present in a special part of their digestive tract.Explanation:Rabbits are herbivores and their feed contains large amounts of cellulose.Their own digestive enzymes cannot break down cellulose directly.They have an enlarged caecum, which houses cellulose-digesting microorganisms.These microbes ferment roughages and release nutrients that the rabbit can absorb.This adaptation makes the enlarged caecum the key organ for roughage digestion.Answer:B. enlarged caecum