Final answer:
The correct answer is option c. The characteristic that makes Silvia Moreno-Garcia's Mexican Gothic a true gothic detective novel is that it focuses on a madman who holds his lover captive.
Step-by-step explanation:
The characteristic that makes Silvia Moreno-Garcia's Mexican Gothic a true gothic detective novel is option C: it focuses on a madman who holds his lover captive. A gothic detective novel typically combines elements of gothic fiction, which includes supernatural and mysterious elements, with a detective story that involves solving a crime.
In Mexican Gothic, the main character faces a mystery involving a seemingly mad and dangerous person who is holding their loved one against their will. This combination of gothic and detective elements makes it a true gothic detective novel.