Answer:
B. A parasite that is threatening the health of coral reefs is the flatworm.
Explanation:
I used the process of elimination to see which statements were too casual.
In A., the phrase "it's a real problem" sounds like something someone could say to their friends when complaining about something, rather than an informative text.
C. says that it "makes life for coral miserable", which I was unsure of, but found the use of miserable to be interesting, since people rarely project emotions onto corals, of all animals. It's almost like personification, which is not a formal way of expressing an idea.
D. says "one major pain", which, like A., is something a friend could say to you when complaining.
Sorry, I know these explanations are sort of vague, since I go by ear, but it's how I would get the answer.
I hope this helps!
