The following passage (paragraphs 21-23) adds to the development of the story mainly by suggesting that Rikki-tikki is easily scared.
In these paragraphs, we see Rikki-tikki's reaction to the hiss from the grass where Nag, the snake is lurking. Rikki-tikki's immediate jump back by two clear feet indicates his surprise and fear in the face of danger.
This reaction helps establish Rikki-tikki as a vulnerable character and adds tension to the story by highlighting the threat posed by Nag.
It sets the stage for Rikki-tikki's character development as he confronts and deals with this new and potentially dangerous adversary which contributes to the overall narrative development of the story.
Therefore, the Option D is correct.