What does the pronoun "it" in "and truly it is (wa innahā) heavy" refer to?

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

1. It refers to the prayer (ṣalāh). [Ibn ʿAbbās, al-Ḥasan, Mujāhid, the majority]
2. It refers to the Kaʿbah and the Qiblah, as the mention of prayer indicates the Qibla. [Al-Ḍaḥḥāk from Ibn ʿAbbās, Muqātil]
3. It refers to seeking help. [Ibn al-Anbārī]