The sentences that contain an example of contested usage are:
C. The chair for the chair of the board of directors is at the head of the board table. (Contested usage of "chair" as a noun and the ambiguity around it) D. The postman needs to know who this letter is addressed to so it can be delivered. (Contested usage of "to" at the end of the sentence)
These two sentences illustrate instances where linguistic preferences or grammatical structures are debated or considered incorrect in formal contexts.