The Lookup() function ' s fourth parameter must be the value to match against the lookup column — here, SubscriberKey in the Fav_Animals data extension — and that value must be sourced from the current recipient ' s own SubscriberKey at send time. In this scenario, because the audience is being pulled from a Salesforce Report rather than a native Marketing Cloud list, the relevant personalization string that resolves correctly is the plain, unscored SubscriberKey personalization string (matching the field/column naming used generally in Report-based and Salesforce Data Extension sends), which is what option C supplies as the literal fourth argument. Option A ' s ContactID is a Salesforce record identifier, not the Marketing Cloud Subscriber Key value the Fav_Animals data extension is keyed on, so it would fail to match. Option B ' s _emailaddr personalization string returns the recipient ' s email address, an entirely different value than SubscriberKey, and would not correctly match the lookup column. Option D ' s underscore-prefixed _subscriberkey is the legacy/system personalization string typically associated with native list-based sends rather than Salesforce Report-audience sends, making the unscored SubscriberKey reference the more contextually correct choice for this Report-driven scenario. Reference: AMPscript Lookup() Function, Personalization Strings for Salesforce Report Audiences.