我觉得更合适的答案应是 D。正如你自己的看法,表示在具体某一天的早上应该用 on。如:
We arrived on the morning of the sixth. 我们在6号早上到达。(《牛津英语语法》))
We got the news on the morning of the wedding. 我们在婚礼举行的上午得到了这个消息。(《牛津高阶英汉双解词典》)
On the morning of July 24th, the suspect left his house at 8:15 a.m.在7月24日的上午,嫌疑犯于8点15分离开了家。(《韦氏高阶英语词典》)