Which of the following are examples of hard corals?

The choice of elkhorn and staghorn corals as examples of hard corals is correct because both belong to the family of reef-building or scleractinian corals, known for their rigid calcium carbonate structures. Hard corals typically create the framework of coral reefs, playing a crucial role in marine ecosystems.

Elkhorn corals have a distinctive branching structure that resembles elk antlers and are important for providing habitat and shelter for a variety of marine species. Staghorn corals also have a branching formation, which provides similar benefits to reef ecosystems. Their ability to grow in a variety of conditions and contribute to reef structures makes them key players in sustaining diverse marine life.

In contrast, the other options include different types of corals or organisms that do not fall under the category of hard corals. For instance, the first choice mentions brain and cup corals, which are hard corals, but the latter choices include soft corals like sea fans and types that do not have a hard skeletal structure, reinforcing why elkhorn and staghorn are the correct representatives of hard corals.