The answer is D. Mule.
A donkey is a domesticated member of the horse family, Equidae. It is smaller than a horse, with long ears and a short tail. Donkeys are often used as pack animals, and they are also popular as pets.
A camel is a large, even-toed ungulate mammal native to Africa, Asia, and Australia. Camels have long necks and legs, and they are adapted to living in hot, dry climates. Camels are often used as pack animals, and they are also popular as tourist attractions.
A cow is a large, hoofed mammal that is raised for its milk, meat, and hides. Cows are members of the bovine family, which also includes bison, yaks, and buffalo. Cows are ruminants, which means that they have a four-chambered stomach that helps them to digest their food.
A mule is a hybrid offspring of a male donkey and a female horse. Mules are sterile, which means that they cannot reproduce. Mules are often used as pack animals, and they are also popular as riding animals.
The word that is different from the rest is mule. Mules are the only hybrid offspring on the list. The other three words are all members of the same family, Equidae.