C7 is referred to as the vertebra prominens because of the presence of the dens its spine is not bifid it has no intervertebral disc between it and the next vertebra its spinous process is larger than those of the other cervical vertebrae

The correct answer is that the vertebra prominens is called as such because its spinous process is larger than those of the other cervical vertebrae. The spinous process of C7 is very prominent that it can be felt on the back of the neck during neck flexion. 

The dens is found at C2 vertebra or the axis. All of the cervical vertebrae has bifid spinous processes. The only verterbrae without intervertebral disc in between is C1 (atlas) and C2 (axis).