The correct answer to this open question is the following.
The influences from China, India, and the Middle East that were imparted on the kingdoms of Indochina and the islands in Southeast Asia were related to culture, traditions, customs, and government.
It is important to remind what are these Southeastern Asia countries that received major influences from China, India, and the Middle East. These were Cambodia, Vietnam, Thailand, Singapore, Malaysia, Philipines, Indonesia, Laos, Brunei, and Myanmar.
With the arrival of merchants, India, China, and the Middle East exerted a heavy influence in those above-mentioned nations. So trade played a key role in the spread of culture and religion. The Sunda Strait and the Strain of Malacca connect the Indian Ocean with the China Sea. Buddhist and Hindu missionaries arrived in Southeastern Asia to evangelize the people of the region.