The correct answer is snippet A, alert('Test');, which is a standard method in JavaScript for invoking a dialog box with specified text content. This method is suitable for testing the insertion of malicious scripts, as it should only trigger when JavaScript is executed within the client's browser context. Snippet B, Alert('Test');, is incorrect due to the use of a capital 'A' in 'Alert', which is not recognized in JavaScript due to case sensitivity. Snippet C, aler't('Test');, contains a misplaced single quote, resulting in invalid syntax, thus, the script would not run. Snippet D, confirm('Test');, would indeed create a dialog box but one that asks for confirmation (OK/Cancel) and is typically used to demonstrate a different form of interaction than just providing an alert.