To answer this question, you need to be familiar with the WMLScript Dialogs Library and its functions.
Let's go through each option to determine which one is not a WMLScript Dialogs Library function:
Option A) confirm() - This is a valid WMLScript Dialogs Library function that displays a dialog box with a message and two buttons, typically used to confirm or cancel an action.
Option B) alert() - This is a valid WMLScript Dialogs Library function that displays a dialog box with a message and an OK button, typically used to display a message or notification.
Option C) prompt() - This is a valid WMLScript Dialogs Library function that displays a dialog box with a message and a text input field, typically used to prompt the user for input.
Option D) error() - This is not a valid WMLScript Dialogs Library function. There is no error() function in the WMLScript Dialogs Library.
Therefore, the correct answer is D. error(). This option is not a WMLScript Dialogs Library function.