When you ask a question, you need to inverse the subject ("ils") and the verb ("rendent").

In this sentence, the verb is "rendre":

Dans quels pays les coureurs se rendent-ils ?

Another example:

Je vais à Paris. (I’m going to Paris)

Vais-je à Paris ? (Am I going to Paris ?)

Another tip: In some cases, you will have to add a "t" between the subject and the verb, except if your verb already finish by a "t", or a "d":

aime-t-il ?

parle-t-elle ?

Sait-il ?

Prend-on ?

More info: https://www.lawlessfrench.com/grammar/euphonic-inversion/