What is the role of alā in “أَلَا إِنَّ الْحَقَّ وَاضِحٌ”?

Aḍrub al-Khabar: Ibtidāʾī, Ṭalabī and Inkārī Reports

• It calls the hearer’s attention
• It assigns nominative case to the noun
• It prohibits an act
• It is emphatic nūn