Your answer will be C) Workplace cliques can cause horizontal disharmony (just like how cliques would form in say a high school!)
We can eliminate the others because A. Office friendships would actually lead to more socializing, B. Ethnic similarities are usually not a big factor in choosing to socialize or be friends with a person, and D) This is not impossible, just usually not encouraged
The correct sentence would be... " I don't understand how we could disagree! We both witnessed the same accident!" You need a capital I and a capital W at the start of your second sentence.