Is it permissible to refuse one's mother, who will not be harmed, if she asks for help in a Quran memorization center that distracts from studies, while fulfilling all her other rights?

May Allah reward you for your eagerness to honor your mother. If your situation is as you described, there is no harm in speaking with your mother and explaining your preoccupation with studies, requesting to be excused from matters related to the Quran memorization center.

If your mother insists, and if undertaking these tasks would cause you harm, then you are not obligated to obey her in that. Rather, you should obey her in what you are capable of, and politely apologize for what you cannot do. However, if you are able to perform these tasks without harm, then you must obey her, even if it involves some difficulty. Ibn Taymiyyah said: "A person is obliged to obey their parents in matters that do not involve disobedience... This applies to what benefits them and causes no harm. If it is difficult for him but does not harm him, then it is obligatory; otherwise, it is not."