Answer:
Dorsal column.
Explanation:
Osmium metal has the ability to binds with the lipid. Osmium is a heavy metal and it turns the area black whenever it binds with lipid.
Dorsal column sense the ability of proprioception, touch and vibration. The dorsal column of lipid is expected to be most dark due to the presence of large lipid composition in the dorsal column.
Thus, the correct answer is option (a).