Final answer:
A groin is the coastal engineering structure that is built perpendicular to the shoreline to trap sand and prevent erosion.
Step-by-step explanation:
The coastal engineering structure built perpendicular to the shoreline is known as a groin. Other structures mentioned, such as breakwaters, seawalls, and jetties serve different purposes.
Groins are designed to trap sand as it is moved down the beach by the process of longshore drift. Breakwaters are usually parallel to the shore and protect against waves. Seawalls are vertical structures that protect the shore from wave action. Jetties are similar to groins but are primarily designed to keep navigation channels free of sediments and protect shorelines at harbor entrances.