Final answer:
The most effective way to ensure a customer understands how to use their digital video service is by having them demonstrate the service's basic operations.
Step-by-step explanation:
The best way to determine if a customer has learned to operate their digital video service is option C) Ask them to demonstrate basic operations. This method provides immediate, practical evidence of their understanding and proficiency. Option A, giving them a written test, may not accurately reflect their practical skills. Option B, returning to the customer's home a week later, is not time or cost-effective for verifying learning. Lastly, option D, gauging by the number of calls received after leaving, does not provide insight into individual customer competency and can be influenced by many factors other than understanding how to use the service.