Solution

Acoelous vertebrae are flat on both the anterior and posterior ends of the centrum (spool in the middle). An example would be the vertebrae in the human spine.
Procoelous vertebrae are anteriorly concave and posteriorly convex. They are found in frogs and modern reptiles.
Amphicoelous vertebra has centra with both ends concave. This shape is common in fish, where most motion is limited. Amphicoelous centra often are integrated with a full notochord.
Opisthocoelous vertebrae are the opposite, possessing anterior convexity and posterior concavity. They are found in salamanders, and in some non-avian dinosaurs.
Therefore, the correct answer is option B.