What was the reason for Abraham's request?

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

1. Abraham (pbuh) saw a dead body being torn apart by insects and wild beasts, prompting him to ask this question. [Ibn ʿAbbās, al-Ḥasan, Qatādah, al-Ḍaḥḥāk, ʿAṭāʾ al-Khurāsānī, Ibn Jurayj, Muqātil] As for what the dead body was, there are three opinions: (i) It was a dead man. [Ibn ʿAbbās] (ii) It was the carcass of a donkey. [Ibn Jurayj and Muqātil] (iii) It was a dead fish. [Ibn Zayd]

1. When he was given the good news that Allah had taken him as an intimate (khalīl), he asked this question to confirm the authenticity of the glad tidings. [al-Suddī via Ibn Masʿūd and Ibn ʿAbbās] It is also reported from Saʿīd ibn Jubayr that when Abraham (pbuh) received this good news, he asked, "What is the sign of that?" and was told, "That Allah answers your prayer and brings the dead to life at your request," which led him to ask this question.
2. He asked in order to remove any whispers of doubt or uncertainty. [ʿAṭāʾ ibn Abī Rabāḥ]
3. After Nimrod argued with him about the resurrection of the dead, he asked to see the thing that he had said concerning Allah (and His power). [Muḥammad ibn Isḥāq]