Medieval Cholas rose to prominence during the middle of the 9th century C.E. and established the greatest empire South India had seen. They successfully united the South India under their rule and through their naval strength extended their influence in the Southeast Asian countries such as Srivijaya. Under Rajaraja Chola I and his successors Rajendra Chola I, Rajadhiraja Chola, Virarajendra Chola and Kulothunga Chola I the dynasty became a military, economic and cultural power in South Asia and South-East Asia. Rajendra Chola I's navies went even further, occupying the sea coasts from Burma to Vietnam, the Andaman and Nicobar Islands, the Lakshadweep (Laccadive) islands, Sumatra, and the Malay Peninsula in Southeast Asia and the Pegu islands. The power of the new empire was proclaimed to the eastern world by the expedition to the Ganges which Rajendra Chola I undertook and by the occupation of cities of the maritime empire of Srivijaya in Southeast Asia, as well as by the repeated embassies to China. They dominated the political affairs of Sri Lanka for over two centuries through repeated invasions and occupation. They also had continuing trade contacts with the Arabs in the west and with the Chinese empire in the east. Rajaraja Chola I and his equally distinguished son Rajendra Chola I gave political unity to the whole of Southern India and established the Chola Empire as a respected sea power. Under the Cholas, the South India reached new heights of excellence in art, religion and literature. In all of these spheres, the Chola period marked the culmination of movements that had begun in an earlier age under the Pallavas. Monumental architecture in the form of majestic temples and sculpture in stone and bronze reached a finesse never before achieved in India.

What area did the Cholas unite in the 9th century?