Ad-Dhikr is to the Qur'an as Ummul-Kitab is to

In the Qur'an, there are certain phrases or verses that are given special significance and are repeated frequently. These phrases are known as ad-Dhikr. Ad-Dhikr helps to emphasize and reinforce important concepts in the Qur'an. It serves as a reminder and a way to keep certain teachings and principles fresh in the minds of the believers.
Similarly, in the Qur'an, there is a chapter that is considered to be the essence and summary of the entire book. This chapter is known as Ummul-Kitab, which translates to "the Mother of the Book" or "the Opening". Ummul-Kitab, also known as Al-Fatihah, is the first chapter of the Qur'an and it holds great significance.
So, to answer the question, the relationship between ad-Dhikr and the Qur'an is similar to the relationship between Ummul-Kitab and Al-Fatihah. Ad-Dhikr is a general term referring to various repeated phrases in the Qur'an, while Ummul-Kitab specifically refers to the first chapter, Al-Fatihah, which is considered the summary and essence of the entire Qur'an.
Therefore, the answer is Fatihah.