The correct answer is: a) Malinithan.
Malinithan is an archaeological site in Arunachal Pradesh that provides evidence of early human settlements. It is located in the Changlang district of the state, near the border with Myanmar. The site was discovered in 1974, and has since been excavated by archaeologists from the Archaeological Survey of India.
The excavations at Malinithan have revealed a number of artifacts, including stone tools, pottery, and human remains. The stone tools are of the Acheulian type, and date to the Lower Paleolithic period. The pottery is of the Neolithic type, and dates to the Upper Paleolithic period. The human remains are of the Homo erectus type, and date to the Middle Paleolithic period.
The discovery of Malinithan provides evidence that Arunachal Pradesh was inhabited by early humans at least as early as the Lower Paleolithic period. This is significant because it shows that the region was not as isolated from the rest of the world as was previously thought.
The other options are incorrect because they are not archaeological sites in Arunachal Pradesh. Bhismaknagar is a historical site, Ita Fort is a military fort, and Tawang Monastery is a Buddhist monastery.