D. Creation Date

Explanation:

Event Time: Represents the time when the event occurred. In the context of Case data, the creation of a case marks the beginning of the event.
Why other options are less suitable:
Last Modified Date: Reflects changes made to the case after its creation, not the initial occurrence.
Resolution Date: Indicates the end of the case, not the beginning.
Escalation Date: Only relevant if the case was escalated, not applicable to all cases.
By using the Creation Date as the Event Time, the data stream accurately captures the point in time when the case was initially created and begins its lifecycle within the system. This is crucial for time-based analysis, such as identifying trends in case creation over time, performing time-series analysis, and building accurate historical views of case data.