Answer:
Rikki-tikki moves the plot forward by trapping nag in the jar.
Explanation:
A protagonist is the leading character or one of the major characters in a drama, movie, novel, or other fictional text, in this case, Rikki-Tikki.
Rikki-Tikki is the protagonist of the excerpt and in the excerpt, it says, "Nag coiled himself down, coil by coil, round the bulge at the bottom of the water-jar, and Rikki-tikki stayed still as death. After an hour he began to move, muscle by muscle, toward the jar."
This shows us that Rikki-tikki trapped nag in the jar, by stating that nag coiled himself down, coil by coil, round the bulge at the bottom of the water-jar.
Hence, the answer to this question is, Rikki-tikki moves the plot forward by trapping nag in the jar.