Final answer:
The atlas (C1 vertebra) lacks a body and is responsible for allowing one to nod the head. It forms the atlanto-occipital joint with the occipital condyles of the skull.
Step-by-step explanation:
The vertebra that lacks a body and helps allow one to nod the head is the atlas (C1 vertebra). The atlas is distinctive because it does not have a body or spinous process, rather, it consists of a ring-shaped structure with an anterior and a posterior arch and elongated transverse processes. It supports the skull and forms the atlanto-occipital joint with the occipital condyles of the skull, which allows for the nodding motion, akin to saying "yes". In contrast, the axis (C2 vertebra) has the unique feature of the dens, that projects upward from the vertebral body and articulates with the atlas, allowing for the rotation of the head.