A series of red flashes directed at an aircraft means respectively in flight on the ground
Why this is the answer
Per the TC AIM tower light signals, a series of red flashes means “airport unsafe, do not land” in flight and “taxi clear of the landing area in use” on the ground.
Reference
TC AIM-RAC – Visual Signals TC AIM-RAC – Visual signals
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.