La risposta è “under”! Se vuoi far pratica per quanto concerne preposizioni e la lingua che effettivamente si parla, al di là dei libri di testo, considera l’idea di dare un’occhiata al mio profilo! 😊

My feeling, as a native speaker, is this - thrive on = you thrive because of the stress; it feeds you. - thrive under = you thrive in spite of the stress; there's stress but it doesn't bother you and you thrive anyway. It's very nuanced and ultimately the difference is so slight that people won't stop to analyse what you mean. Hope that helps.

Thrive under stress.

We say 'to thrive *under* stress'.