I love my mother very much, and want to do good deeds on her behalf now that she has passed away. Is it better to fast for a month each year and dedicate its reward to my mother, or to feed thirty poor people for that month and gift its reward to her? Is it necessary to feed thirty individuals, or can I instead give food to as many families as possible? Is it permissible to designate a fixed amount of my money each month and gift its reward to her? Also, what is the minimum amount at which zakat on wealth becomes obligatory?

Everything you wish to do in this regard is permissible and sanctioned in Islamic law. It is established in Islamic law that actions whose benefit extends to others are more virtuous and earn greater reward than those that benefit only oneself. Therefore, if you are able to choose either between feeding the poor and fasting, then feeding the poor is better, as its benefit extends to others.

As for the minimum amount at which zakat on wealth becomes obligatory (nisab), it is the market value of 85 g of 21-karat gold at the time zakat becomes due, i.e., after the completion of one lunar year.

And Allah Almighty knows best.