Which of the following must be true at 500 feet above TDZE during an approach?

At 500 feet above the touchdown zone elevation (TDZE) during an approach, it is crucial that the aircraft is operating at the calculated approach speed within specified limits. This ensures optimal control of the aircraft and allows for a stable final approach. Maintaining the appropriate approach speed is essential for handling changes in wind, potential gusts, and ensuring that the aircraft can safely execute the landing.

Flying at the correct approach speed helps ensure adequate lift and prevents stall conditions, particularly important during this phase of flight when precision is key. Deviating from the specified limits could compromise the aircraft's performance and lead to unsafe conditions upon landing.

The other options present situations that either do not apply universally or would not be advisable during this critical phase of flight. For instance, having engines spooled at reduced thrust can be appropriate in some scenarios, but it's not a requirement at this height. A descent rate of at least 1000 feet per minute could be excessive or inadequate depending on the specific circumstances, and flight at any airspeed is not safe because it disregards the critical relationship between speed, lift, and control.