Which type of reef is found offshore and is separated from the mainland by a lagoon?

The type of reef that is found offshore and is separated from the mainland by a lagoon is a barrier reef. Barrier reefs typically develop parallel to the coastline but are at some distance from it, allowing for the formation of a lagoon between the reef and the shore. These reefs are often larger and more extensive than fringing reefs, which are directly attached to the shoreline, and they play a critical role in protecting coastlines from wave action and erosion while also providing diverse habitats for marine life.

In contrast, the other types of reefs mentioned do not share this specific characteristic. Fringing reefs are located adjacent to the land, typically with no lagoon, while patch reefs occur within lagoons or on the continental shelf and are smaller. Coral atolls, while also found offshore, are typically circular or oval and enclose a lagoon, but they have their own distinct formation process and are not classified as barrier reefs.