Which of the following is not a form of shirk according to the Quran?

According to the Quran, monotheism is not a form of shirk. Shirk refers to the act of associating partners or idols with Allah, which is considered a grave sin in Islamic belief.
Monotheism, on the other hand, is the belief in and worship of only one God. It is the fundamental concept in Islam, and Muslims are taught to believe in the oneness of God, known as "Tawheed."
The Quran repeatedly emphasizes the importance of monotheism and condemns the act of associating partners with Allah. This concept is summarized in the declaration of faith in Islam, the Shahada, which states, "There is no God but Allah, and Muhammad is His messenger."
In contrast to monotheism, atheism is the belief that there is no God or higher power. While atheism does not involve associating partners with Allah, it goes against the central pillar of Islamic belief in the existence of a single God.
Therefore, the correct answer is atheism as it stands in opposition to the belief in the oneness of God and does not involve associating partners with Allah.