The statement that is true about barrier islands is that "They protect the mainland from the effect of waves on its shore."
Barrier islands are described as a form of coastal areas which is usually flat or have some bumpy areas full of sand, known as a dune system.
Barrier islands are known to create substantial protection for the mainland from storm effects as they are situated near coral reefs and mangroves.
Hence, in this case, it is concluded that the correct answer is option C. "They protect the mainland from the effect of waves on its shore.
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