The type of third-party activity fired independently from any other SAS activity (added in the "Then" section of an activity rule) is called:

Other 3rd party activity" is the correct answer because this category is specifically designed for third-party tags or pixels that need to fire independently of standard retargeting or conversion tracking. These activities are typically added in the "Then" section of an activity rule, indicating they are triggered when the rule's conditions are met. "3rd party retargeting tracking activity" and "3rd party conversion tracking activity" are specific types of third-party activities with predefined purposes (retargeting users or tracking conversions, respectively). While they are also third-party activities, "Other 3rd party activity" is the broader, more general category for any third-party script that doesn't fit neatly into the retargeting or conversion buckets but still needs to be executed based on a rule.