The correct answer is option 'C': Automobile.

Detroit, located in the state of Michigan, is famously known as the "Motor City" or "Detroit City" due to its historical significance in the automobile industry. It has been a major hub for the production of automobiles in the United States for over a century. Several factors have contributed to Detroit's association with the automobile industry.

1. Historical Significance: Detroit's association with the automobile industry dates back to the early 20th century when Henry Ford founded the Ford Motor Company in 1903. This marked the beginning of the mass production of automobiles, with Ford's Model T becoming the first affordable car for the average American. Other automobile manufacturers such as General Motors and Chrysler also established their headquarters in the Detroit region, solidifying its reputation as the center of the American automobile industry.

2. Manufacturing Hub: Detroit became a prominent manufacturing hub for automobiles due to its strategic location and proximity to key resources. The city is situated on the Great Lakes, providing easy access to shipping routes for raw materials like steel, rubber, and glass. Additionally, the region had a skilled workforce, which was crucial for the production of automobiles on a large scale.

3. Innovation and Technology: Detroit has been at the forefront of automotive innovation and technology. The region is home to numerous research and development centers, engineering firms, and automotive design studios. This concentration of talent and expertise has led to significant advancements in automotive technology, including the development of electric and autonomous vehicles.

4. Economic Impact: The automobile industry has had a profound economic impact on the Detroit region. It has been a major source of employment, providing jobs to thousands of people. The industry's success has also contributed to the growth of other sectors, such as steel, rubber, and electronics, which are essential for automobile manufacturing.

Overall, Detroit's association with the automobile industry has made it famous worldwide. The city has played a pivotal role in shaping the automotive landscape and continues to be a significant player in the industry. Its legacy as the "Motor City" has left an indelible mark on the history of American manufacturing and innovation.