ok so natural means like without people's infuence
if it happens in nature (like in forrests) without people making it, then it is natural
clouds are obvouosly not man made (we actually have found out how to make clouds but that's not the point)
oceans are natural (duh, they have been there for long time)
plastics are not natural, we have to mix ingredients togehter that normally wouldn't be together to make plastic
animals are obvously naturall
answer is C. plastics