What is the feminine of bachelor?

In English there are no masculine or feminine forms. English uses gender specific nouns for a male or a female.

The gender specific noun for a male is a bachelor.

The noun 'spinster' is a gender specific noun for a female who is past a child bearing age but has never married.

The noun 'bachelor' is the noun used for an unmarried, adult male of any age. However, at the time the Word 'spinster' was commonly used, no corresponding noun for a male was used because an unmarried man of any age is not past an age for (the expectation of) siring children.

Old timers will speak of a 'bachelor lady.' The Word bachelorette is a more recent addition to popular culture, because the Word spinster has such negative connotations.