Final answer:
The Mughal ruler during Guru Nanak Dev Ji's time was Babar, who founded the Mughal Empire in India. Babur's rule overlapped with the lifetime of Guru Nanak, the first Guru of Sikhism. Babur was followed by Humayun, Akbar, and Jahangir in the Mughal dynasty.
Step-by-step explanation:
The reigning Mughal ruler during Guru Nanak Dev Ji's time was Babar. Guru Nanak, the founder of Sikhism, was born in 1469 and lived during the period when Babar established the Mughal Empire in India after his victory in 1526. Although Babar and Guru Nanak's lives overlapped, their direct interactions remain a matter of historical discourse rather than well-documented encounters. Babur was succeeded by his son Humayun, and the lineage continued with notable rulers such as Akbar and Jahangir, each making distinct contributions to the Mughal dynasty.
To address the additional question about Guru Tegh Bahadur, he was a leader of the Sikh religion and was executed by the Mughal emperor Aurangzeb, who was known for his less tolerant stance towards religions other than Islam.