Final answer:
The correct option is c. The process of Islamization during the early Abbasid period involved religious conversion, cultural assimilation, and tolerance towards non-Muslim communities.
Step-by-step explanation:
The process of Islamization during the early Abbasid period involved a combination of religious conversion, cultural assimilation, and tolerance towards non-Muslim communities. Forced conversion of all subjects (option a) was unlikely to be successful because Islam was a minority religion and conversion would result in a loss of tax revenue for the caliphate.
Instead, the Abbasids allowed non-Muslims to maintain their religious practices and imposed a tax called jizya (option c) on non-Muslims, which was higher than the tax for Muslims. The process of Islamization also involved cultural conversion, with aspects of Arab and Islamic culture being adopted by indigenous populations while retaining their own religious communities.