From the given sentence, form a feminine noun from the masculine noun given:The author was on the verge of a new idea.

Also, if twenty candidates are sent from B to A, the number of students in A is double the number of students in B;

Feminine noun is the noun that represents the female equivalent of any word.

The given word Author means anyone who writes or composes anything.

Considering all the given options -

A: Writer - Writer means anyone who writes and hence it is a common gender noun.

B: Writeress - This is a wrong word in itself.

C: Poetess - Poetess represents any female who compose poems and it is a female equivalent of word Poet.

D: Authoress - Authoress is any female who writes or composes something and hence it is the female equivalent of Author.

The correct answer option is D i.e., Authoress