The alert recipients tab specifies where notification messages should be sent when alerts are triggered or cleared. The options that can be used are:

Invoke a webhook URL. This option allows you to send a HTTP POST request to a custom URL that can perform various actions based on the alert information. For example, you can use a webhook to create a ticket in a service desk system, post a message to a chat channel, or trigger another workflow1

Send an SMS message. This option allows you to send a text message to one or more phone numbers when an alert is triggered or cleared. You can customize the message content and format using variables and templates2

Send to email addresses. This option allows you to send an email notification to one or more recipients when an alert is triggered or cleared. You can customize the email subject, body, and attachments using variables and templates. You can also include information from search results, the search job, and alert triggering in the email3

Therefore, the correct answer is A, C, and D.

1: https://docs.splunk.com/Documentation/Splunk/latest/Alert/Webhooks 2: https://docs.splunk.com/Documentation/Splunk/latest/Alert/SMSnotification 3: https://docs.splunk.com/Documentation/Splunk/latest/Alert/Emailnotification