Final answer:
b. Bhai Mardana Ji, a Muslim and a lifelong companion of Guru Nanak Dev Ji, accompanied him with a rebeck, contributing significantly to the early Sikh music traditions.
Step-by-step explanation:
The Muslim who accompanied Guru Nanak Dev Ji with a rebeck (a musical instrument invented by himself) was b. Bhai Mardana Ji. Bhai Mardana Ji was a lifelong companion of Guru Nanak, the founder of Sikhism, and he played a significant role in the early development of Sikh music traditions.
He is widely remembered and revered in Sikh history for his deep friendship with Guru Nanak and his contributions to the faith. Guru Nanak Dev Ji established Sikhism in the 15th century CE, a religion that combines elements of Hinduism and Islam and emphasizes equality among all humans, rejecting the caste system prevalent at that time in India.