In the spinal cord, white matter is separated into ascending and descending tracts organized as

White matter in the spinal cord is named as such because of it's paler color than the other areas which are darker (gray matter). The light color is due to the presence of myelin, a fatty substance which speeds up the rate that electrical impulses travel to and fro the neurons.
Spinal white matter is separated into ascending and descending tracts organized as vertical columns within the cord. There are many names for these specific tracts, usually based upon where the signals traveling in a specific column began, and where it leads to. Such as the Spino-Thalamic tract, which starts in the spinal cord and travels to the Thalamus of the brain. This gives us a uni-directional flow of information, from bottom (spine) to top (brain), so the Spino-Thalamic tract is an ascending column of white nerves.
On the other hand, the Cerebro-Spinal tract descends, from the cerebral of the brain down the spinal cord to reach different parts of the body.

Your answer is b. codominant