After accepting a clearance and subsequently finding that it cannot be complied with, a pilot should
Why this is the answer
This is the lost-intent rule under CAR 602.31: take whatever immediate action safety requires, then advise ATC as soon as possible. ATC's clearance assumed you'd fly it, so they must be told when you can't — saying nothing is unsafe.
Reference
CARs 602.31 – Compliance with Air Traffic Control Instructions and Clearances TC AIM-RAC – ATC Clearances, Instructions and Information
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.