Final answer:
The line that signals the beginning of the narrative is: "When she was nine, my daughter Kathleen asked if I had ever killed anyone."
Step-by-step explanation:
The line that signals the beginning of the narrative within the frame story in "Ambush" is: O When she was nine, my daughter Kathleen asked if I had ever killed anyone. This line introduces the conversation between the narrator and his daughter, setting the stage for the storytelling that follows.
Learn more about Narrative within a frame story