Final answer:
Prince Muhammad Muazzam, the third son of Aurangzeb, claimed the throne as Bahadur Shah I after Aurangzeb's death, marking the start of the Mughal Empire's decline.
Step-by-step explanation:
The third son of Aurangzeb who proclaimed himself the Emperor of India after the death of Aurangzeb was Prince Muhammad Muazzam. He took on the reigns of the Mughal Empire following a war of succession with his brothers. Upon his ascension, he assumed the regnal name Bahadur Shah I, and his reign marked the beginning of the decline in Mughal power as the empire started to break apart amidst internal conflicts and the rise of regional powers.