Answer : Runners-up
The plural form of the word runner-up is runners-up. The plural form of a hyphenated compound word is done by pluralizing the main element of the word. If the main element of the word is not a known, the second element can be pluralized. However, in this case the first element is runner and that is noun meaning we pluralized the runner into runners making it "runners-up". Other examples of pluralized hyphenated compound word are fathers-in-law, lookers-on, editors-in-chief and a lot more.
