The necessary conditions for the growth of coral reefs are outlined in the options provided.

Option 1 includes three conditions: 1) photic conditions, which means that the reefs need to be in shallow water where sunlight can penetrate to allow photosynthesis by algae living in the coral; 2) clean and sediment-free water, as excessive sedimentation can smother the coral and hinder their growth; 4) tropical sea water with a temperature of 20 to 21°C, as coral reefs thrive in warm waters.

Option 2 includes only two of the necessary conditions for coral reef growth: clean and sediment-free water, and tropical sea water with a temperature of 20 to 21°C. This option is incorrect as it misses out on the importance of photic conditions.

Option 3 includes two conditions: photic conditions and sea salinity of 6%. This option is incorrect as the sea salinity requirement is not a necessary condition for coral reef growth.

Option 4 includes all four necessary conditions for coral reef growth: photic conditions, clean and sediment-free water, sea salinity of 6%, and tropical sea water with a temperature of 20 to 21°C. However, the correct answer is 1, indicating that only options 1,