Is it permissible to give Zakat to the grandfather for the purpose of getting dentures, taking into account that he has a small pension and married children, and it is unknown whether they assist him, and is my assistance to him a religious obligation?

There is no contradiction between the two answers; both affirm that it is not permissible for a child to give the zakat from his wealth to his ascendants (fathers, grandfathers, mothers, and grandmothers) as long as their maintenance is obligatory upon him. However, if their maintenance is not obligatory upon him, then it is permissible to give them from the zakat if they are among those eligible, such as a maternal grandfather whose expenses are not sufficient for him and whose children are negligent in providing for him. In this case, his maintenance is not obligatory upon the grandchild.