A. Vertebrates are advanced chordates having cranium (brain box) around the brain and hence, they are called as craniata. So, all vertebrates have skull i.e. cranium.
B. Their body consists of serially arranged vertebrae which comprises of a major part of their axial exoskeleton, the vertebral column or the backbone.
C. Their skin covered by protective skeleton that comprises of scales, feathers, hair, etc.
D. They have an endoskeleton which may be made up of cartilage or bones.
Hence, statement A is correct. So, the correct answer is 'Presence of skull'.