Concept:An allusion is an indirect reference to a famous person, event, or literary work.Explanation:The phrase "habitual Thomases" refers to "Doubting Thomas," the biblical disciple who refused to believe without proof.By calling the family members "habitual Thomases," the writer compares them to that well-known doubting figure.This indirect reference is therefore an allusion, not an allegory, aphorism, or apostrophe.Answer:C. allusion