Final answer:
Queen Victoria was the last monarch before Queen Elizabeth II to celebrate a diamond jubilee, marking 60 years on the throne in 1897.
Step-by-step explanation:
The last monarch before Queen Elizabeth II to celebrate a diamond jubilee was Queen Victoria. She celebrated her diamond jubilee in 1897 after 60 years on the throne. Neither William IV, Edward VII, nor George VI reached this milestone. William IV did not reign long enough for a diamond jubilee, while Edward VII and George VI passed away before they could reach a 60-year reign. Therefore, the correct answer to the question 'Who was the last monarch before the Queen to celebrate a diamond jubilee?' is C. Victoria.