Where does the motor neuron of the autonomic nervous system exist?
A. Posterior vagal trunks
B. Autonomic ganglia
C. Vagus nerves
D. Thoracic nerves

Hint: Motor neurons are cells found in the brain and spinal cord that permit us to move, speak, swallow and breathe through conduction of instructions from the mind to the muscle groups that perform those functions. Their nerve fibers are the longest withinside the body. They may stretch from the base of the spinal cord to the entire hind limbs.

Complete answer:
Option A - Posterior vagal trunks are found by the vagus nerves in the thorax and are nowhere associated with the autonomic nervous system.
Thus, this option is not correct.
Option B - The motor neurons of the autonomic nervous system are found in the autonomic ganglia. An autonomic ganglion consists of clusters of cytons and dendrites of neuronal cells. It acts as a junction between the autonomic nerves from the CNS and the target organs. Hence, motor neurons of the autonomic nervous system exist in the autonomic ganglia.
Thus, this option is correct.
Option C - Vagus nerve is the tenth cranial nerve and is associated with the parasympathetic control of the heart, lungs and the digestive tract. It comprises both motor and sensory nerve fibers but is not the site of the existence of motor neurons of the autonomic nervous system.
Thus, this option is not correct.
Option D - thoracic nerve fibers supply neural connections in the upper half of the body and the motor neurons of the autonomic nervous system are not found here.
Thus, this option is also not correct.

Therefore, Option B. Autonomic ganglia is the correct answer.

Note:
The autonomic nervous system regulates involuntary processes along with coronary heart rate, blood pressure, respiration, digestion, and sexual arousal. It comprises three anatomic divisions: sympathetic, parasympathetic and enteric. The parasympathetic and the sympathetic divisions are complementary in function to each other.