How do you respond to someone who says the hijab is a sunnah and optional to wear?

Scholars have differed regarding the obligation of covering the face, and the preponderant opinion is that it is obligatory. As for covering the rest of the body, it is obligatory by consensus. The hijab (definition) is purity and protection from indecency, and a sign of chastity. It is a divine decree in which believing women have no choice. Ostentatious display (tabarruj) is a characteristic of the pre-Islamic era of ignorance (Jahiliyyah) and the people of Hellfire; it spreads indecency in society and is hypocrisy and wantonness. Repentance and wearing the hijab are obligatory before it is too late, for death is near and recompense is coming.