The PIC of an aircraft shall comply with any light signals or ground marking prescribed in the CARs
Why this is the answer
Per CAR 602.31, the PIC must comply with prescribed light signals and ground signals at all times, provided safety is not jeopardized. That safety qualifier is the key — the correct answer is the conditional one, not the unconditional 'at all times.'
Reference
TC AIM-RAC – ATC Clearances, Instructions and Information CARs 602.31 – Compliance with Air Traffic Control Instructions and Clearances
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.