The secondary characters are those who interact directly with the protagonists, who are the main characters of the story and the focus of the narrative.
As we can see when reading the question above, Bernice and Marjorie are the focus of the text's narration, because the whole narration revolves around the two. The characters Roberta, Warren and Otis have the ability to interact with the two protagonists, but without turning the attention of the narration to any of them, therefore being secondary characters.
The answer is "stacks." That is the object of the phrase, and since a person is searching for something ON the shelf, they aren't searching for the shelf itself.
