Answer:
When the United States paid $20 million, as set out in the Treaty of Paris, is the right answer.
Step-by-step explanation:
A treaty between the United States and Spain that put an end to the Spanish-American War came to be known as the Treaty of Paris. This treaty was signed on 10th December 1898. Following the terms and conditions of the treaty, Spain abandons its sovereignty over and title to Cuba and Puerto Rico, Guam, and the Philippines to the U.S. At the same time, the United States gave a sum of $20 million to Spain as compensation.