Question

fill in the blank question
the groove indicated by the letter a on the figure is the posterior
sulcus.

Explanation:

In anatomy (a subfield of Biology under Natural Science), the posterior median sulcus is a groove on the spinal cord. Given the context of a groove (sulcus) and the term "posterior", the likely term to fill in is "median" as the posterior median sulcus is a known anatomical structure.

Answer:

median