It is benefactor. There is no "feminine" form because all English nouns are of common gender. If you feel the need to point out that the benefactor is female person, it is possible to use the stilted word benefactress, but not advisable, since it is grammatically unnecessary and besides it is thoroughly sexist to draw attention to the gender of an antecedent.
The feminine form of benefactor is benefactress.
Benefactor is technically gender neutral, but benefactress is a specifically feminine form of the word.
A benefactor is a person who gives another person a gift, usually in the form of money to some form of charity or organization.
The feminine form of ami is amie. The feminine plural is amies.
The feminine form of alumnus is alumna. The feminine plural is alumnae.
The feminine form of bajo is baja. The feminine plural is bajas.
Mistress is the feminine form of master. It is already in feminine form.
Baroness is the feminine form.
The feminine form of charmant is charmante. The feminine plural is charmantes.
The feminine form for host is "hostess."
The benefactor of the scholarship is unknown.
The feminine form of "mauvais" is "mauvaise."