You are presented with the question "What is secreted by organisms like frogspawn to become a reef?" and the following answer choices: 
- exocrine glands
 - food and shelter
 - exoskeleton
 - bones
 - Energy
 - Chemical energy
 - plasma
 - Energy.

Now knowing that many corals secrete an exoskeleton that builds up to become a coral reef and good examples are Bubble coral, Elegance coral, Frogspawn and other Euphyllia corals, choose the best answer.


 

   exoskeleton      