Which of the following group of nerves are blocked first in spinal anaesthesia?

When the local anesthetic is injected into CSF of subarachnoid space, the nerve roots are affected first. Blockade of neural transmission in the posterior nerve root fibers interrupts somatic and visceral sensation, whereas blockade of anterior nerve root fibers prevents efferent motor and autonomic outflow. The autonomic preganglionic fibers are more sensitive than somatic motor fibers which are less sensitive than somatic sensory fibers. Ref: Butterwoh IV J.F., Mackey D.C., Wasnick J.D. (2013). Chapter 45. Spinal, Epidural, & Caudal Blocks. In J.F. Butterwoh IV, D.C. Mackey, J.D. Wasnick (Eds), Morgan & Mikhail's Clinical Anesthesiology, 5e.