What are the irregular lines on the skull marking former growth areas called?

The correct choice refers to sutures, which are the irregular lines on the skull that indicate the boundaries between the different bones of the skull. These sutures are formed as the bones of the skull grow and fuse together during development. They are fibrous joints that allow for some flexibility in the skull, which is essential for accommodating brain growth in infants and young children. Over time, sutures become more defined and may appear as jagged lines on the surface of the skull.

Other terms listed serve different purposes: fontanels are the soft spots on an infant's skull where the sutures have not yet fused, allowing for compression during birth and subsequent brain growth. Parietals refer to the parietal bones of the skull, which are one of the several bones that make up the cranial structure but do not specifically denote growth areas. Similarly, soft spots typically describe the fontanels rather than the connections or growth areas indicated by sutures.