When an engine starts up and the starter key is released, to what position does the key return?

When an engine is started with a starter key, it typically operates in a momentary position that engages the starter motor. Once the engine has started, and the key is released, it naturally returns to a position that allows normal engine operation. In most cases, this would be the "On" position where the vehicle is ready for operation but does not engage the starter.

However, in some vehicles, especially those equipped with modern electrical and ignition systems, the key may also have a position that allows for auxiliary functions or "Boost." This can be vital for systems that require power even when the engine is not running, such as radio or other accessories. Hence, the statement that the key returns to both the "On" and "Boost" positions can be seen as accurate for those vehicles.

In summary, the correct interpretation is that upon releasing the starter key, it returns to a position that allows for normal vehicle operation while potentially still permitting auxiliary features to be accessed, justifying the rationale behind the answer that includes both aspects.