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

Concerning whom this āyah was revealed, there are three opinions:

1. It was revealed regarding those to whom it was said, "Eat of what is on earth lawful and good" (al-Baqarah: 168). Thus, the pronouns "them" refer back to these individuals. [Muqātil]
2. It was revealed concerning the Jews, and this is a new topic. Therefore, the pronouns "them" refer to something not previously mentioned. [Reported by Ibn Isḥāq from Ibn ʿAbbās]
3. It was revealed about the Arab polytheists and the disbelievers of Quraysh, with the pronouns referring to the statement, "And among mankind there are those who take besides Allah rivals" (al-Baqarah: 165).

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As for "that which Allah has revealed", depending on the three opinions above:

1a. Based on the first opinion, the referent of "that which Allah has revealed would be lawfulness of the lawful and the prohibition of the unlawful. 

2a. According to the second, it is Islam. 

3a. According to the third, it is monotheism and Islam.

Concerning whom was this revealed? And what is meant by "that which Allāh has revealed"?