Final answer:
The U.S. government coding system used to classify firms by business operations is known as SIC, or Standard Industrial Classification, although it has largely been supplanted by the NAICS system.
Step-by-step explanation:
The abbreviation for the U.S. government coding system that classifies a firm by its specific type of business operations is D. SIC, which stands for Standard Industrial Classification.
The SIC system is used to categorize businesses based on their industry and has been a vital part of understanding economic data and patterns. However, it's worth noting that the SIC has been largely replaced by the NAICS (North American Industry Classification System), which provides a more updated framework for classifying business establishments.