The "Custom" notification setting provides granular control over which events trigger a notification for a specific repository. This allows a user to subscribe to particular categories of events, such as "Security alerts," while ignoring others like "Issues" or "Pull Requests." This is the only option that directly addresses the need to receive specific notifications that explicitly include security alerts without subscribing to all repository activity.

A. Ignore: This setting mutes all notifications for the repository, which is the opposite of the user's requirement.

B. Participating and @mentions: This only sends notifications for conversations the user is directly involved in or mentioned in, and would not guarantee alerts for all security events.

C. All Activity: This option is too broad, as it sends notifications for every update in the repository, not just the specific security alerts the user wants.

1. GitHub Docs, "Configuring your watch settings for a single repository." This official documentation details the available notification levels for a repository. It explicitly states that under the "Custom" option, a user can "select the types of events you want to be notified of," and lists "Security alerts" as one of the selectable event types.

2. GitHub Docs, "Configuring notifications." This document provides a comprehensive overview of notification settings. In the section "Choosing your watch settings," it describes the "Custom" level as the method to "choose to be notified of specific events," reinforcing its suitability for the scenario.