Which search string would only return results for an event type called success ful_purchases?

This is because event types are added to events as a field named eventtype, and you can use this field as a search term to find events that match a specific event type. For example, eventtype=successful_purchases returns all events that have been categorized as successful purchases by the event type definition. The other options are incorrect because they either use a different field name (tag), a different syntax (Event Type:: or event type—), or have a typo (success ful_purchases). You can learn more about how to use event types in searches from the Splunk documentation1.