What can be used in ArcGIS Desktop to display the most current observation for each airplane without data modification?

The correct answer involves using a tracking layer with the Tracking Analyst extension in ArcGIS Desktop. A tracking layer is specifically designed to manage and display time-based data without modifying the underlying dataset. It allows users to visualize real-time or historical data, showing each airplane's most current observation based on the tracking information stored in the dataset.

Tracking layers utilize an efficient way of displaying dynamic information, such as the movement of airplanes over time, while providing tools to analyze movement patterns. With this approach, real-time updates can be displayed, maintaining the integrity of the original dataset and allowing for a more accurate representation of the data as it evolves.

Other options may not correctly address the requirement of displaying the most current observation without modifying the data. For instance, while time layer animation uses the Animation toolbar and can visualize time-based changes, it modifies data for display purposes and is typically not as efficient in managing real-time observations as a tracking layer. Similarly, a time-enabled feature layer does visualize time data but may not capture changes with real-time accuracy as effectively as a tracking layer. The network dataset from the Network Analyst extension focuses on modeling transportation networks and doesn’t relate directly to displaying observations of airplane data based on time, thus missing the context of the question.