Answer: Bailan.
Step-by-step explanation:
We need to complete the sentence "Ellos ___ bien" with the correct conjugation of the verb "bailar"
Now, is easy to see that the verbal person is "ellos", and conjugating with the options given we have:
The given conjugations are (in the present tense):
Baila (this is for "El/ella/usted baila"
Bailamos (this is for "nosotros bailamos")
Bailo (this is for "yo bailo")
Bailan (this is for "ellos/ellas bailan" this is the correct option)
Ellos bailan bien