Answer:
Las hadas vivían felices.
Step-by-step explanation:
To complete this exercise, you have to select the sentence that has a regular verb in the imperfect tense.
The Spanish imperfect tense ("pretérito imperfecto" in Spanish) is used to talk about past habitual actions or to talk about what someone was doing but then got interrupted with something else.
"Vivir" is a regular verb. In the imperfect tense, all the verbs that end with "-ir" in the third person plural form are conjugated with "-ían", so the verb is "vivían".