The ground idle blast danger area extends back from the tail of a medium size jet aeroplane for at least
Why this is the answer
Per the TC AIM-AIR jet-blast danger figures, a medium-size jet's ground-idle blast danger area extends at least 450 ft behind the tail — less than a jumbo's 600 ft but more than an executive jet's 200 ft. Idle thrust alone still produces a hazardous downstream blast.
Reference
TC AIM-AIR – Jet and Propeller Blast Danger
Question reproduced verbatim from Transport Canada’s PSTAR Study and Reference Guide (TP 11919E). The answer key is Transport Canada’s; the explanation is ours.
A study aid — not a substitute for the official guide, the CARs, or your instructor.