The sentences containing an example of contested usage are:
C. And William also said that he did not understand why he couldn't play in the football team anymore. (The word "in" could be contested as some may argue it should be "on" in this context.)
D. The postman needs to know who this letter is addressed to so it can be delivered. (The phrase "who this letter is addressed to" is contested as some argue it should be "to whom this letter is addressed.")
These two sentences contain phrases where grammatical correctness may be debated.