What is the "offering which is easy"?

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

1. It means a sheep. [ʿAlī ibn Abī Ṭālib, Ibn ʿAbbās, al-Ḥasan, ʿAṭāʾ, Ibn Jubayr, Ibrāhīm, Qatādah, al-Ḍaḥḥāk]
2. It means whatever is available from camels and cattle only. [Ibn ʿUmar, ʿĀʾishah, al-Qāsim]
3. It means according to one's ability. This view has been reported by Ṭāwūs from Ibn ʿAbbās. It has also been narrated from al-Ḥasan and Qatādah that they said, "Its highest is a camel, its middle is a cow, and its lowest is a sheep."