A raptor at hack should be taken up when...

Taking a raptor up from hack when it starts hunting demonstrates an understanding of the bird's natural instincts and behavior. Hack is a method of training where a raptor is allowed to acclimate to its surroundings and develop its hunting skills independently. When the bird begins to hunt, it indicates that it is becoming confident and proficient in its ability to find and capture prey, which is a crucial step in its development.

This timing is essential, as it aligns with the goals of falconry—promoting the raptor's natural abilities while ensuring it is ready for a more structured training regimen. If the bird is taken up too soon or before it demonstrates its hunting behavior, it may not have fully developed the skills necessary for effective hunting when returned to the falconer’s care.

The other options point to specific times or durations that do not directly correlate with the bird’s readiness based on its behavioral maturity and instinctual development, making them less relevant to the appropriate timing for taking up a raptor at hack.