An opponent can always choose not to sacrifice a nonland permanent or discard a card (and therefore lose 4 life) even if they have cards in their hand or nonland permanents on the battlefield.

If Rottenmouth Viper leaves the battlefield before its triggered ability resolves, use the number of blight counters that were on it as it last existed on the battlefield to determine how many times each opponent must make the listed choice.

In a multiplayer game, each opponent in turn order makes all of their choices, then all of the actions occur simultaneously. Opponents will know what choices opponents earlier in turn order made, although cards chosen to be discarded this way won’t be revealed until they’re discarded at the end of the process.

While resolving Rottenmouth Viper’s triggered ability, your opponent chooses a card to be discarded without revealing it, chooses a nonland permanent to be sacrificed, or chooses to do neither. Cards or permanents they’ve chosen previously during this process can’t be chosen again. Then that player repeats that process if it hasn’t yet been done once for each blight counter on Rottenmouth Viper. Finally, that player discards the chosen cards, sacrifices the chosen permanents, and loses 4 life the appropriate number of times.