To answer this question, you need to understand the definitions of each term. Let's go through each option to understand why it is correct or incorrect:
Option A) An error - An error refers to a human mistake or incorrect action that leads to a deviation from the desired or expected behavior. It may or may not be visible to the end-user. Therefore, this option is incorrect.
Option B) A fault - A fault refers to a defect or malfunction in a system or component that causes it to deviate from its expected behavior. It is typically not visible to the end-user. Therefore, this option is incorrect.
Option C) A failure - A failure refers to a situation where a system or component does not perform its intended function or deliver the expected results. It is visible to the end-user and indicates that the system is not working correctly. Therefore, this option is correct.
Option D) A defect - A defect refers to a flaw or imperfection in a system or component that causes it to deviate from its expected behavior. It may or may not be visible to the end-user. Therefore, this option is incorrect.
The correct answer is C) a failure. This option is correct because when a deviation from specified or expected behavior is visible to the end-user, it is referred to as a failure.