The correct answer is Polonius's violent death.
In Hamlet, Ophelia already had a weakened mental state, which only deteriorated after her father was violently murdered by none other than her love, Hamlet. She was already drifting into madness as a result of Hamlet's aggressive behavior, but when he accidentally killed her father, Polonius, it only aggravated her mental state, causing her to ultimately kill herself.
Ophelia is not the only one with a demented state of mind - almost everyone's mental state declines, until they all die at the end of the play.