What should the questioner do regarding the unpaid zakat for his father who passed away 12 years ago, and is he obliged to pay all that is outstanding, or can he pay this year's zakat and pay the arrears in installments?

The ownership of the aforementioned father's wealth transfers to his heirs immediately upon his death. Therefore, whoever inherited a nisab or less, and had other wealth that, when combined, reached the nisab, must pay zakat on it every year from the day of its possession. The delayed zakat must be paid immediately because it is an obligatory right that does not lapse with time, and it is not permissible to be lenient, delay, or pay in installments in its disbursement.