Final answer:
The recognizing feature of the vertebra prominens is its long and prominent spinous process located at the base of the neck.
Step-by-step explanation:
The recognizing feature of the vertebra prominens, also known as C7 vertebra, is its long and prominent spinous process. This spinous process can be easily felt at the base of the neck, at the back of the body. It is longer than the spinous processes of the other cervical vertebrae.