What is the ruling on saying "Yarhamuk Allah" to one who sneezes, and is it sufficient for one of those present to say it, or is it obligatory upon everyone? And does the same ruling apply to returning the greeting of Salaam?

Sneeze-replying (tashmit al-'atis) is a Sunnah or a communal obligation (fard kifayah) according to the majority of scholars, while some consider it an individual obligation (fard 'ayn). So, whoever hears someone praise Allah (al-hamid) should say to him: "Yarhamuk Allah (May Allah have mercy on you)," and the sneezer should reply to him: "Yaghfir Allahu lana wa lakum (May Allah forgive us and you)" or "Yahdikum Allahu wa yuslihu balakum (May Allah guide you and rectify your affairs)," with the latter being preferable. As for giving the greeting of peace (salam), it is a Sunnah, and returning it is a communal obligation.