I think it should be D. A loss in biodiversity due to catastrophic event.
Syntax is also collectively known the arrangement of words in a sentence. The correct answer to your questino is B. For that reason it isn't possible for any other answer in this case to be correct because B actually denotes a rather formal way of how you could talk about syntax.
I just took the test, the answer is weathering.
Riparian zones are important to the natural environment because they function as natural biofilters and protect the aquatic life and the aquatic environment.
