What we can infer about Sara's character based on this excerpt from the novel is "Sara was very mature for her age"
There are some keywords that lead to that inference, for example, It would have been an old look for a child of twelve, and Sara Carter was only seven, imagination cannot be seen, this is part of her features which show that she had the personality of an adult.
The other options have no fundament in this excerpt, the word that they use is thinking not imagining, this could be confused with dreaming.
The other to options talk about social interaction and all the excerpt is about her won.