Final answer:
The quote belongs to the climax of the plot line in the story 'Rikki-tikki-tavi' where the big man saves Rikki-tikki-tavi by shooting and killing Nag, the dangerous snake.
Step-by-step explanation:
The quote, "The big man had been awakened by the noise, and had fired both barrels of a shotgun into Nag just behind the hood," belongs to the climax of the plot line in the story "Rikki-tikki-tavi." The climax is the turning point in the story where the conflict reaches its peak and the tension is at its highest. In this particular quote, the big man steps in to save Rikki-tikki-tavi by shooting and killing Nag, the dangerous snake. This action resolves the conflict and leads to the resolution of the story.