Animals which have a backbone are called
A. Mammals
B. Amphibians
C. Vertebrates
D. Invertebrates

Hint: Chordates are the organism which have backbones. They have a muscular system that mainly consists of a central nervous system partly enclosed within the backbone.

Complete answer:
Animals which have skeletons and backbone are called Chordates or vertebrates. There are different kinds of vertebrates which includes humans, mammals, birds, amphibians, reptiles and fishes.
Characteristics of Vertebrates:
-There is a distinct similarity in the anatomy of vertebrates with the presence of vertebral column, gastrointestinal tract and spinal cord.
-Notochord develops into a vertebral column, and the vertebrae are present on the dorsal side.
-Presence of the central nervous system is a crucial identifier. Nerve tube of the spinal cord in the anterior part expands into the brain.
-Jaws are present in some vertebrates
-Internal skeleton aids in the distribution of different muscle attachment nodes.

Classification of Vertebrates include:
-Class Mammalia: Mammals have four chambered hearts and warm blood. They give birth to their offspring. Mammary glands are present for feeding the younger ones. Fertilization is usually internal. Examples-Kangaroo, dolphin.
-Class Aves: Birds fall under this class of vertebra. The distinctive feature of Aves is the feature covered body and forelimbs that evolved as wings of flight. Example- Pigeon, crows
-Class Reptilia: The body of reptiles are covered by scales. Reptiles have three chambered hearts and are cold blooded animals. Examples- tortoise,turtle
-Class Pisces: This includes fish with scales all over the body. These vertebrates are oviparous and respiration takes place through the gills. Fishes have two chambered hearts and the skeleton is composed of cartilage.
-Class Amphibia: These are aquatic and terrestrial animals and the body is covered by moist skin without the scales. Respiration is through lungs and gills and the heart is three chambered. Example: Salamander, toad

Hence, the correct answer is option (C).

Note: Vertebral column is a defining characteristic feature of vertebrates in which the notochord is present in the phylum.
Chordata is replaced by a segmented series of stiffer elements that is separated by mobile joints. Gills or lungs are present in vertebrates for the purpose of respiration.