The correct choice would be: The use of a subplot increases the sense of mystery.
By intertwining the detective’s romantic storyline with the central murder investigation, the author creates layers of intrigue. The subplot allows for deeper character development and emotional stakes, while also keeping readers engaged with the main plot. The relationship between the detective and librarian not only adds personal tension but also heightens the overall mystery as their interactions can provide insights or red herrings that might affect the case. The dual narratives keep readers guessing while enriching the story’s emotional depth.