Explanation
According to the Salesforce documentation, the variable tag that must be included in an email to allow prospects to manage their email preferences is %%unsubscribe%% or %%email_preference_center%%. A variable tag is a placeholder that is replaced with a specific value when the email is sent. The
%%unsubscribe%% variable tag is replaced with a link that allows the prospect to unsubscribe from all emails from the sender. The %%email_preference_center%% variable tag is replaced with a link that allows the prospect to opt in or out of specific email lists or categories. Either of these variable tags can be used to enable prospects to manage their email preferences, and they are required by law in some countries. %%opt_out%% or %%email_preference_center%%, %%opt_out%% report_spam%%, and %%unsubscribe%% or
%%opt_out%% are not valid variable tags that can be used to allow prospects to manage their email preferences, as they are either incorrect or nonexistent. References: Salesforce documentation