Which of the following fields is NOT a default field extracted by Splunk?

The correct answer is user_id because it is not a default field that Splunk extracts from data. Default fields, like _time, host, and sourcetype, are automatically extracted during the indexing process and are available for every event in Splunk. These fields provide essential information about the event timestamp, the source of the data, and the type of data source.

In contrast, user_id is typically considered a custom field that may be created when you have specific data that contains user identification. It depends on the configuration of the data inputs, and must often be explicitly defined through field extractions or defined in the Splunk configuration files. Thus, while user_id can be extracted and used in your searches and analyses, it is not one of the fundamental fields that Splunk automatically provides.