Explain what happened in the bathroom, and who marked Nag's back with a mark and why?I want the explanation of what happened in the bathroom.
Many religions have creation myths that explain how parts of nature came to be as they are. The Hindu religion tells the story of Brahma, the "creator god," who "put his mark upon all our people when the first cobra spread his hood to keep the sun off Brahm...as he slept." Snakes are honored in the Hindu religion, and this mark was very likely a sign that the animal had been blessed by Brahma because of the service it performed for this chief god.
Rikki-Tikki-Tavi's presence in the bungalow did not bode well for Nag and Nagaina as the snake and the mongoose are mortal enemies. To ensure their place and safety for their own children, Nag and Nagaina create a plan to kill the family, thus ridding themselves of the pet mongoose. Rikki overhears this plot and prepares himself to protect the family and live up to the duty of a mongoose. When Nag falls asleep after hiding in the bathroom to wait for "the big man," Rikki attacks Nag. He locks onto him, refusing to let go. The noise from their struggle awakens the man, who shoots and kills Nag, setting the stage for the inevitable showdown with Nagaina, who blames Rikki for her mate's death.