Final answer:
The best way the people of London use the Thames to relax is through cruising and boating, as this activity offers scenic pleasure and leisure directly on the water, which is a popular and iconic aspect of London life.
Step-by-step explanation:
The best illustration of how the people of London use the Thames to relax is D) Cruising and boating. This activity encapsulates the leisure aspect of the river, allowing locals and tourists alike to enjoy the scenery and the pleasure of being on the water. Cruises along the Thames can vary from short trips to longer excursions, which might include dining and sightseeing opportunities of London's iconic landmarks.
While fishing along the banks and swimming and diving may also be considered recreational, they are less representative of the common leisure activities associated with the Thames today. Taking educational tours does provide an opportunity to relax and learn, but it emphasizes education over leisure.
In contrast, cruising and boating directly align with the idea of relaxation and the enjoyment of the river's atmosphere. This is also supported by historical and contemporary depictions of the Thames, which often show boats and cruises as popular features among the bustling activity of London.