What is "the middle prayer" mentioned in this āyah and why is it named this way?

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

1. It refers to the ʿAṣr prayer. Muslim reported in his "Ṣaḥīḥ" from the narration of ʿAlī (may Allah be pleased with him) that on the day of the Battle of the Trench, the Prophet (ﷺ) said, "They distracted us from the middle prayer, the ʿAṣr prayer; may Allah fill their houses and graves with fire." It is also narrated from Ibn Masʿūd, Samurah, and ʿĀʾishah that the Prophet (ﷺ) said, "It is the ʿAṣr prayer." This is the view of many authorities including Abū Ḥanīfah and the Hanbali school.
2. It refers to the Fajr prayer. This view is held by ʿUmar, ʿAlī in one narration, Abū Mūsā, Muʿādh, Jābir ibn ʿAbd Allāh, Abū Umāmah and various others, includin Mālik and al-Shāfiʿī. Abū al-ʿĀliyah reported: "I prayed the dawn prayer with the companions of the Messenger of Allah (ﷺ) and then asked them, 'Which is the middle prayer?' They replied, 'The one you just prayed.'"
3. It refers to the Ẓuhr prayer. This is reported from Ibn ʿUmar, Zayd ibn Thābit, Usāmah ibn Zayd, Abū Saʿīd al-Khudrī, ʿĀʾishah in one narration, and it is narrated by Ḍumayrah from ʿAlī who said, "It is the Jumuʿah prayer, which is the Ẓuhr prayer on other days."
4. It refers to the Maghrib prayer. [Narration from Ibn ʿAbbās; Qabīṣah ibn Dhuaib]
5. It refers to the ʿIshāʾ prayer. [ʿAlī ibn Aḥmad al-Nīsābūrī]

Regarding the meaning of "al-wusṭā" (middle), there are three views:

(a) It means the middle prayer in terms of length. This would apply to Maghrib, since the shortest obligatory prayers are two rakʿah (Fajr), and the longest are four rakʿahs (Ẓuhr, ʿAṣr, ʿIshāʾ). \ (b) It means the best prayer. The term "middle" also means "the best and most balanced," as in His saying, "Thus We have made you a middle nation" (2:142). If we interpret "al-wusṭā" to mean the best, each school of thought may claim that their interpretation is correct.\ (c) It means the middle prayer in terms of timing. This also allows for multiple views, as follows:

Those who say it is ʿAṣr can argue that it comes between two prayers during the day and two prayers at night, making it the middle. Those who say it is Fajr argue that it is between night and day. Those who say it is Ẓuhr argue that it is the middle of the day. Those who say it is Maghrib argue that Ẓuhr was the first prayer made obligatory, making Maghrib the middle. Those who say it is ʿIshāʾ argue that it is between two prayers that cannot be shortened (Maghrib and Fajr).