Statement I tells us that Sanjay returned to Hyderabad from Mumbai on Thursday. However, this does not tell us the exact day Sanjay visited Mumbai.
Statement II tells us that Sanjay left Hyderabad on Monday for Mumbai. With this, we can determine that Sanjay must have visited Mumbai on Tuesday, as the return date is Thursday.
Both statements are needed to answer the question, as one provides the departure day and the other provides the return day, allowing us to infer the day of the visit.