Which of the following is an example of a human-made barrier?

Seawalls are a prime example of a human-made barrier designed specifically to protect coastal areas from wave action and prevent erosion. These structures are constructed along shorelines and serve to absorb and reflect the energy of incoming waves, helping to safeguard both coastal communities and infrastructure from the impacts of storms and high tides.

In contrast, dunes, wetlands, and mangroves are natural features that play roles in coastal ecology and protection but are not the result of human construction or intervention. Dunes are formed by the action of wind on sand, wetlands occur naturally in areas where water saturates the soil, and mangroves grow in coastal intertidal zones, providing habitat and protection but arising naturally without human interference. Therefore, among the options provided, only seawalls fit the definition of a human-made barrier.