Option d is the correct answer. Saurashtra was a part of the Gupta empire after Chandragupta II defeated the Saka king Rudrasimha III and annexed his territories in Gujarat. Saurashtra was an important region for trade and commerce, as it had access to the Arabian Sea and ports like Bharuch. Statement a is correct: Forced labour (Vishti) became more common than before in this period. Vishti was a form of tax paid by the people in the form of labour or service to the state or the landlords. It was prevalent in some regions of the Gupta empire, especially in south India. Statement b is correct: A passage in the Vishnu Purana refers to the Guptas enjoying all the territories along the Ganga up to Prayaga. The Vishnu Purana is an ancient Hindu text that contains various myths and legends. It also mentions some historical events and dynasties, including the Guptas. It praises the Gupta rulers for their conquests and piety. Statement c is correct: The Mehrauli inscription suggests that Chandragupta fought against a confederacy of enemies in Bengal and also led a campaign into the Punjab. The Mehrauli inscription is an iron pillar erected by Chandragupta II, also known as Vikramaditya, in Delhi. It records his military victories over various enemies, including a confederation of kings in Bengal and some rulers in Punjab.