From which airspace boundary is IFR flight mandatory to FL600?

• A. Base of high level controlled airspace to FL600
• B. FL180 and above
• C. Above, but not including FL180
• D. FL180 to FL600 inclusive

Answer: (A)

IFR flight is required inside controlled airspace, and high‑level controlled airspace has a specific lower boundary that marks where that requirement begins. From the base of the high‑level controlled airspace up to the top of that space, up to FL600, IFR operation is mandatory. So the correct boundary to start IFR requirements is the base of the high‑level controlled airspace, not a fixed altitude like FL180. The other options imply a boundary at or above a particular level, whereas the rule depends on where the high‑level controlled airspace begins in that region.