The Extensions Management tool in QRadar is primarily used for managing extensions, which include custom content packages that can enhance or update QRadar's functionality. This makes option C correct, as QRadar can be updated by using the Extensions Management tool to install new extensions or update existing ones. Other options are incorrect because adding a log source is not typically managed through the Extensions Management tool, the tool can be used to export content, and importing CSV extensions is not a common feature associated with it.