Question

which of the following means once upon a time?
hace muchos siglos
de repente
había una vez

Explanation:

"Once upon a time" is a common phrase used to start stories. In Spanish, "Había una vez" is the equivalent phrase used to begin stories, similar to how "Once upon a time" is used in English. "Hace muchos siglos" means "Many centuries ago" and "De repente" means "Suddenly", neither of which have the same storytelling - initiating meaning as "Once upon a time".

Answer:

Había una vez