Which of the following is an example of hard engineering in coastal defense?

Sea walls are a prime example of hard engineering in coastal defense. They are constructed structures made of concrete or stone designed to protect the coastline from erosion and to prevent flooding from the sea. By creating a barrier between the land and the water, sea walls absorb and reflect wave energy, helping to minimize the impact of storm surges and high tides.

In contrast, beach nourishment involves adding sand to beaches to replace lost sediment, which is considered a soft engineering approach aimed at enhancing the beach's natural defenses. Vegetation planting focuses on the use of plants to stabilize soil and reduce erosion by acting as a natural barrier, which again falls under soft engineering techniques. Coastal zoning regulations are administrative practices aimed at managing land use near coastal areas, but they do not involve physical alterations to the coast itself.

Thus, among the provided options, sea walls clearly represent a hard structural response to coastal erosion and management challenges.