Explanation:

• Coral reefs have been necessary to sustain life in the oceans. These reefs not only provide sanctuary to a vast ecosystem but also protect the coastlines from eroding.
• Coral reefs are formed when the freely moving larvae of corals attach themselves to sedimentary rocks or hard surfaces near the coastlines. This process is undertaken with the help of several other processes like sedimentation, compaction, cementation and solidification of the skeletons of coral polyps.
• The slight salt in ocean waters is extremely important for the development of coral polyps. The polyps extract calcium from the waters to protect their skeletons. Hence, mild salinity is a necessity for coral reefs to flourish.

Hence only statement 1st is correct.