The ground idle blast danger area extends back from the tail of a jumbo jet aeroplane for at least
Why this is the answer
Per the TC AIM-AIR jet-blast danger figures, even at ground idle a jumbo jet's blast danger area extends at least 600 ft behind the tail. The larger the aircraft, the longer the idle-blast zone — well beyond the 200–450 ft of smaller jets.
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.