Final answer:
Tabes dorsalis most commonly affects the thoracic levels of the spinal cord.
Step-by-step explanation:
The spinal cord level that is most commonly affected by tabes dorsalis is the Thoracic levels. Tabes dorsalis is a condition that is caused by neurosyphilis, which is a late stage of syphilis infection. It primarily affects the thoracic region of the spinal cord, leading to symptoms such as pain, weakness, and loss of coordination.