Breakdown of Elle a eu un frisson en entrant dans la grotte humide.
Questions & Answers about Elle a eu un frisson en entrant dans la grotte humide.
A eu is the passé composé of avoir.
It is used here because the sentence describes a single completed event: she suddenly felt a shiver at that moment. French often uses the passé composé for this kind of one-time action or reaction.
If you used avait, that would be the imparfait, which would suggest more of an ongoing state or background description, not a sudden reaction.