In an emergency requiring the use of an ELT, it should be turned on
Why this is the answer
An ELT exists to lead search-and-rescue to you, so per the TC AIM-SAR ELT operating instructions, in a real emergency turn it on immediately and leave it on. Cycling it off to save battery or waiting for an ETA defeats the alert it broadcasts on 121.5/406 MHz.
Reference
TC AIM-SAR – ELT Operating Instructions (Emergency Use)
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.