Answer:
Letter C is the correct answer.
Step-by-step explanation:
In the statement quoted in the question (meaning that Gussie "has lost his head over a creature"), the speaker chooses some very particular words for Aunt Agatha. First of all, the expression (lost one's head) indicates that Gussie has lost self-control because of "a creature". The last word is very important because it means that for Aunt Agatha the object in question is no more than it, an object that does not deserve to be called by its name, showing even more disdain.