Is a young man's marriage to a specific girl, despite his mother's objection to it—and the consent of the rest of the family—considered disobedience (عقوق), especially since the young man promised the girl marriage and they had known each other beforehand, and the mother claims that she will not be pleased with him if he marries her?

Filial piety towards one's mother is among the most obligatory duties and greatest acts of devotion, while disobeying her is one of the gravest sins. I advise you to perform Istikhara regarding marriage to the girl. Strive to convince your mother, seeking help from someone who has influence over her, and use every permissible means, coupled with supplication (Du'a). If she agrees, then praise be to Allah. But if she insists on refusing, then obey her, as long as no harm results to you from this. Islam does not approve of a relationship between a man and a woman before marriage; the prescribed path is engagement through her guardian. "And perhaps you dislike a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not." (Quran 2:216) "So be patient; indeed, Allah does not allow to be lost the reward of those who do good." (Quran 11:115)