The most impressive platform which was located on one of the highest point in the Vijayanagara city, used for the religious rituals was:

Mahanavami Dibba

The correct answer is Option (2) → Mahanavami Dibba

Located on one of the highest points in the city, the “mahanavami dibba” is a massive platform rising from a base of about 11,000 sq. ft to a height of 40 ft. There is evidence that it supported a wooden structure. The base of the platform is covered with relief carvings. Rituals associated with the structure probably coincided with Mahanavami (literally, the great ninth day) of the ten-day Hindu festival during the autumn months of September and October, known variously as Dusehra (northern India), Durga Puja (in Bengal) and Navaratri or Mahanavami (in peninsular India). The Vijayanagara kings displayed their prestige, power and suzerainty on this occasion. The ceremonies performed on the occasion included worship of the image, worship of the state horse, and the sacrifice of buffaloes and other animals. Dances, wrestling matches, and processions of caparisoned horses, elephants and chariots and soldiers, as well as ritual presentations before the king and his guests by the chief nayakas and subordinate kings marked the occasion. These ceremonies were imbued with deep symbolic meanings. On the last day of the festival the king inspected his army and the armies of the nayakas in a grand ceremony in an open field. On this occasion the nayakas brought rich gifts for the king as well as the stipulated tribute.