The correct answer is A. FortiSIEM supports bulk rule operations for selected rules. The FortiSIEM 7.4 User Guide states that if you have permission to activate a rule, you can activate or deactivate multiple rules with a single click. The procedure instructs the user to go to Resources > Rules, click the edit icon, select Multiple Rules, choose the rules, and then use the Select Actions panel. In that panel, the guide states that you can select a Severity from the Severity drop-down list to change the selected rules, and you can also select or deselect active status options for new or existing organizations to make the selected rules active or inactive. This proves that both operations are available: severity changes and activation/deactivation changes. Option B is too restrictive because FortiSIEM allows multiple-rule selection. Option C is incomplete because activation/deactivation is also supported. Option D is incomplete because severity changes are also supported. Therefore, the correct answer is that you can change severity and activate or deactivate multiple selected rules.