The correct answer is 'Detailed documentation of the evidence,' because maintaining comprehensive and accurate records of the evidence gathered during an incident investigation is a legal requirement. This facilitates potential legal actions and supports compliance with regulatory standards. Gathering evidence properly ensures that it can be admissible in court, thereby protecting the organization's legal interests. 'A graph of affected systems' does not necessarily relate to legal obligations, as it is more pertinent to the technical analysis of the incident. 'A list of system patches' is important for remediation but is not the foremost priority from a legal standpoint. 'Recommendations for policy improvement' addresses organizational policy rather than specific legal requirements.