What is meant by "righteousness" (birr) 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 adhering to their book; they commanded others to follow it yet did not adhere to it themselves.
2. It refers to following Muḥammad (ﷺ). [Ibn ʿAbbās]
3. It refers to charity; they encouraged it but were stingy themselves. [Al-Zajjāj]