The correct answer is option 2, Chaityas. Pillared halls where Buddhist monks worshipped were known as Chaityas. Stupas, option 1, are dome-shaped structures containing Buddhist relics. Viharas, option 3, are monastic dwellings or residential buildings for monks. Mathas, option 4, refer to Hindu monastic institutions or monasteries. It`s important to note that while all the options are associated with religious or spiritual practices, the specific term used for the pillared halls of Buddhist worship is Chaityas.