The platform used for ritual purposes by the kings of the Vijayanagara Empire was called the Mahanavami Dibba. The Mahanavami Dibba was a grand structure built on multiple levels, located within the royal enclosure of the empire. It was used specifically during the celebration of the Navaratri festival, which was a nine-day long celebration of the goddess Durga.

Option 2, Lotus Mahal, is incorrect as it refers to a different structure within the Hampi complex of the Vijayanagara Empire. The Lotus Mahal was a beautiful pavilion used as a royal palace for the queen and her attendants.

Option 3, Hazara Rama, is also incorrect as it refers to the Hazara Rama Temple, which was a temple dedicated to Lord Rama. It was not specifically used for ritual purposes by the kings.

Option 4, Virupaksha, is incorrect as it refers to the Virupaksha Temple, which was a temple dedicated to Lord Shiva. While it was a significant religious site, it was not the platform used for ritual purposes by the kings.

Therefore, the correct answer is option 1, Mahanavami Dibba.