The principal source for the Arab invasion of Sind in the early eighth century is:

Correct option is C

The Chach-Nama is the most important and detailed source for the Arab invasion of Sind in the early eighth century. It is a historical account of the conquest of Sind by Muhammad bin Qasim and the preceding Hindu rule under Raja Dahir. Originally written in Arabic and later translated into Persian, the Chach-Nama gives a narrative of the military expedition, administrative arrangements, and early Islamic rule in the region.
Hence, the correct answer is (c) Chach-Nama.