Mientras la gente de la Florida tomaba el sol en la playa, la gente de Argentina esquiaba en las montañas.
Explanation:
If we are speaking of simultaneous activities, both verbs must share the same tense as to indicate they are or were occurring simultaneously. The reason the first option is incorrect is because, while grammatically correct, it say that while one of the groups did the one thing, the other did a different one, unlike the third choice that states they both engaged in different activities simultaneously.
