Final answer:
Longshore drift is enhanced by the placement of groins, which are structures designed to interrupt sediment movement and trap sand, thereby increasing deposition in certain areas while potentially causing erosion in others down-drift.
Step-by-step explanation:
The subject of the question is longshore drift, which is a geographical phenomenon that occurs in the nearshore zone. Longshore drift involves the movement of sediment along the coast by a longshore current. This current runs parallel to the shore due to waves hitting the shore at an angle. The correct answer to the question is option d) Is enhanced by groin placement. Groins are structures built out into the water perpendicular to the coastline to trap sand moving in the longshore current, thus enhancing longshore drift in the area behind the groin. However, while groins enhance longshore drift in one area, they can also starve areas down-drift from sand, leading to erosion.
In contrast to the other options: a) is incorrect because longshore drift does not act like an offshore river, b) is not accurate as longshore drift can exist in cliffy regions though the effect might be different, and c) is also incorrect because longshore drift is not reduced by waves—in fact, it is driven by waves.