Heaviest vertebrae in man is 

• In mammals the notochord is converted into a vertebral column which contains the vertebrae. 
• These are bones that support the spine of the mammals and help them in moving forward. 
• The vertebrae are connected to each other with the help of joints called facets. 
• The vertebrae in humans consist of cervical , thoracic, lumbar, sacrum and coccyx. Out of these the lumbar vertebrae which is made up of five bones is the heaviest. 
• It is attached to the nearby muscles and it helps in the forward as well as the backward bending of the spine. 

Hence, the correct option is C.