All messages that were generated for all components on the current page.
If an <apex:message> or <apex:messages> component is not included in a page, most warning and error messages are only shown in the debug log.
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A. B. C. D.A.
The correct answer is A. apex:messages.
In Apex and Visualforce Controllers, the apex:message and apex:messages components are used to display error, warning, or confirmation messages to the user. If an apex:message or apex:messages component is included in the page, all messages generated by the components on the page will be displayed in that component.
However, if an apex:message or apex:messages component is not included in the page, most warning and error messages will only be shown in the debug log, which can make it difficult for users to identify and resolve issues.
Therefore, it is important to include an apex:message or apex:messages component in your Visualforce pages to ensure that users are able to see any error or warning messages that are generated by the components on the page.
Option B, C, and D are not correct because they are not related to displaying error or warning messages in Visualforce pages.