Which script(of indian language) is base for many south-east asian language?

The Grantha script, which evolved from the Brahmi script, was indeed influential in the development of several Southeast Asian scripts. The Pallava script (a variant of Grantha) served as the basis for scripts like Thai, Lao, Khmer, and Burmese. While Tamil and Sanskrit were culturally influential, the Grantha/Pallava script was the direct scriptural ancestor for many Southeast Asian writing systems.