Final answer:
To extend a vacation, the Reservations tab in the Guide Portal should be used, as it manages booking details.
Step-by-step explanation:
To extend Darren's expiring week for his dream vacation using the Guide Portal, you would use the Reservations tab. This tab is typically responsible for managing booking details, including altering the length of a stay or changing the dates of a reservation. The other tabs, such as Billing, Feedback, and Maintenance, serve different functions. Billing would handle any payment details, Feedback might be used to gather client opinions or concerns, and Maintenance is likely concerned with physical upkeep or technical issues related to the service or property.