Final answer:
The mint mark 'D' on US coins indicates that they were minted in Denver, also known as the Mile High City. This corresponds to option A on the question.
Step-by-step explanation:
The letter on US coins that indicates it was minted in the Mile High City, which is Denver, Colorado, is 'D'. Therefore, the answer to the question is A) D. The Denver Mint is one of the United States Mint facilities where coins are produced for circulation. Each US Mint facility that produces coins has a unique mint mark: 'D' for Denver, 'S' for San Francisco, and 'P' for Philadelphia. There is no mint mark for coins produced at the West Point Mint, while 'CC' typically marks coins from the historic Carson City Mint.