Which of the below is a valid and correct mutator method for 'author' (String) attribute of 'Book' class? : a.public String setAuthor() { return author; } b.private void setAuthor(String author) {...

Which of the below is a valid and correctmutator method for 'author' (String) attribute of 'Book' class?

a.public String setAuthor() { return author; }

b.private void setAuthor(String author) { this.author = author; }