A testable acceptance criterion must be specific, measurable, and verifiable, which makes option C the correct choice. Stating that "the response time to confirm a customer submission must not exceed 5 seconds" provides a clear, quantifiable measure that can be tested to determine whether the criterion has been met. This contrasts with more subjective criteria, such as the system being "easy to use," which cannot be easily quantified or measured in a straightforward manner. The specificity of the response time allows for the creation of test cases that can confirm whether the system meets this particular requirement, thus making it a testable acceptance criterion.