The correct answer is 'Adversary Infrastructure', as in the Diamond Model of Intrusion Analysis, prioritizing the understanding of adversary infrastructure is crucial when evidence of data exfiltration is present. This helps analyze the adversary’s command and control servers and the mechanisms used to extract data. Understanding Victim and Capability can supplement this analysis but are not as directly relevant to the context as identifying the infrastructure utilized in the cyber attack. 'Adversary's Tools' without focusing on infrastructure does not provide comprehensive insight into the data exfiltration process.