Bhaskaravarman ruled Kamarupa — ancient Assam — as the contemporary and ally of Harshavardhana, so option 2 is correct.

Who he was. Bhaskaravarman (reigned about 600–650 CE) was the last and greatest ruler of the Varman dynasty of Kamarupa, with his capital at Pragjyotishpura, near modern Guwahati. He came to the throne at a time when Sasanka of Gauda dominated eastern India, and his answer to that pressure was diplomatic: he allied himself with Harsha of Kannauj, and between them they broke Sasanka’s power.

The evidence for the alliance. Two independent sources record it. Banabhatta’s Harshacharita describes the embassy Bhaskaravarman sent to Harsha and the gifts that accompanied it. The Chinese pilgrim Xuanzang (Hiuen Tsang) travelled to Kamarupa at Bhaskaravarman’s invitation and has left an account of the kingdom; he then accompanied the king to the great assemblies Harsha convened at Kannauj and Prayag. His Nidhanpur copper-plate inscription, which records a grant of land to Brahmanas, is the principal epigraphic record of his reign.

Why option 4 fails. “More than one of the above” requires at least two correct names, but Dahir belongs to Sindh in the eighth century and Aswad Jahan is not an Assamese ruler at all, so only a single option survives.

Hence, the answer is Bhaskar Varman.