Rabbits are herbivorous animals, which means they feed on plants. However, plants contain complex carbohydrates, such as cellulose, that cannot be easily digested by rabbits. To overcome this problem, rabbits have an enlarged caecum, which is a part of the digestive system that is responsible for breaking down tough plant materials. The caecum is home to a population of bacteria that help to ferment and break down cellulose into simpler compounds that the rabbit can digest. Therefore, the correct option is "enlarged caecum".