What is "people" (al-nas) referring to in this āyah?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. It refers to all of the descendants of Adam. [Majority]
2. It refers to Adam alone [Mujāhid]. Ibn al-Anbārī explains that this is permissible because the Arabs sometimes use a plural form to refer to a single individual. The meaning of the verse would then be: "Adam was of one religion, but his children differed after him."
3. It refers to Adam and his children, who were initially on the truth, but they differed after Cain killed Abel. [Ibn al-Anbārī]