Which of the following is classified as a basic feature measure?

The classification of basic feature measures in synchro skating often relates to the fundamental elements that demonstrate a skater's balance, control, and fluidity. The Spread Eagle is a classic maneuver that embodies these qualities due to its requirement for a stable position on the blades while gliding smoothly across the ice. It is relatively straightforward, making it accessible for a wide range of skaters, which aligns with the idea of basic features that serve as foundational skills in synchro skating.

In contrast, the other options present more complex elements that require a higher level of technical proficiency or specific body positions that may not be considered basic. For example, the Unsupported Spiral and the Biellmann Spiral involve significant body positioning and balance challenges that go beyond basic skating skills. Similarly, the Upright Extension entails a level of executed extension and balance that is typically associated with advanced skating techniques rather than foundational skills. Thus, identifying the Spread Eagle as a basic feature measure emphasizes its role as a core element within synchro skating.