The internal auditor's objective in sorting the data by vehicle and identifying instances of refueling on the same or sequential dates is most likely to determine whether fuel purchases were legitimate and for work-related purposes. By analyzing patterns of refueling, the auditor can identify any anomalies or unusual activity that may suggest misuse or personal use of the organization's vehicles. This helps ensure that organizational resources are being used appropriately and that there are no instances of fraud or abuse.
Reference:
The Institute of Internal Auditors (IIA) Standard 1220 - Due Professional Care: "Internal auditors must apply the care and skill expected of a reasonably prudent and competent internal auditor." IIA Practice Guide on "Data Analytics and Continuous Auditing"