D) Teddy's father, because he tries to protect Teddy
Process of Elimination:
A cannot be correct, causing danger does not make someone a protagonist, and if anything makes them an antagonist.
B cannot be correct, because, sure, he is the one in trouble, but he's not the main character. For example, in Mario games, peach is the one in danger, but peach isn't the main character. Mario is, because we follow Mario and he is the one that tries to save peach.
C Cannot be correct either. Rikki-tikki did solve the problem, which is obviously very helpful to the protagonist, but still, they are not the protagonist themselves. Imagine a story where someone has a problem, but a different character gives the solution. The person that had the problem is still the main character, the other character just helped the main character.
Hope this helped!