Answer: D

Explanation:
The correct answer is “Customer database.” A primary source refers to original, direct sources of information or evidence. In the context of researching for a business report, a customer database provides firsthand data that is directly gathered from the customers, making it a primary source. Other options like accounting data, audit investigation, and market research agencies might serve as secondary sources or tools for analysis, while “Bing” is a search engine, not a direct source of original information.