The correct answer is b) sulcus.
A sulcus is a shallow groove found on the surface of the brain’s cortex, which is the outer layer of the brain involved in numerous high-level functions, such as thought and action. The sulci (plural of sulcus) help to increase the surface area of the cortex, making it possible for the brain to have more neurons and perform more complex tasks. In contrast, a gyrus is the raised portion between the sulci, while a fissure typically refers to a deeper groove in the brain. Understanding these terms is fundamental when studying brain anatomy.