An on-line commercial site such as Amazon.com is an example of a(n) ________ .

This option is correct. An online commercial site like Amazon.com is an example of a multiuser database application. It allows multiple users to simultaneously access the database to browse products, make purchases, manage accounts, and perform other e-commerce activities.

Option A: single-user database application
This option is incorrect. A single-user database application is designed for scenarios where only one user accesses the database at a time. It is not suitable for large-scale e-commerce sites like Amazon.com.

Option C: e-commerce database application
This option is partially correct but not as precise as Option B. An e-commerce database application specifically refers to applications supporting e-commerce activities, which often involve multiple users concurrently accessing the database.

Option D: data mining database application
This option is incorrect. A data mining database application focuses on analyzing large datasets to discover patterns and insights, rather than supporting e-commerce transactions like Amazon.com.

Conclusion:
An online commercial site such as Amazon.com is an example of a multiuser database application, making Option B: multiuser database application the correct answer.