The correct statement is - A. The Arab Empire unified feuding tribes under the religion of Islam.
The emergence of the new faith, Islam, on the Arabian Peninsula was the key factor that contributed to the unification of the multiple feuding tribes in the region, Under the Islam, they all worshiped the same God, had the same holly book, and had the same prophet in Muhammad, and this was more than enough that all of the tribes unite, use the situation where the Sassanid and Byzantine Empires were military and financially exhausted, and managed to spread on vast space, making one of the biggest empires, and using it to spread out the Islam as well.