Final answer:
The small grand duchy that completes a European trip, as mentioned in the question, is Luxembourg. It is renowned for its economic stability and vital role in the EU.
Step-by-step explanation:
The grand duchy referenced in the question "Our trip to Europe won't be complete without a visit to the small grand duchy" is Luxembourg (Option A). Luxembourg is a small, landlocked country nestled between Germany, France, and Belgium. It stands out for its stable and prosperous economy and is notable for Luxembourg City, its vibrant capital. Despite its size, Luxembourg plays a significant role in the European Union and offers rich cultural sites, making it an essential stop for any comprehensive European tour.