Why does the subject أَخُو in أَخُو حَامِدٍ صَادِقٌ, 'Ḥāmid's brother is truthful,' end with wāw?

Practical Exercises with Phrases in Nominal Sentences

• Because it is one of the five nouns, and when nominative as a muḍāf it ends with wāw
• Because it is dual
• Because it is plural