The correct answer is:
a. huyeron
In Spanish, "ladrones" is a plural noun, so the correct verb form to use with it is also plural. "Huyeron" is the past tense of the verb "huir" (to flee) in the third person plural form, which matches the subject "los ladrones." This means that the correct sentence is "Los ladrones huyeron de la policía," indicating that the thieves fled from the police.
