Pilots operating VFR en route in uncontrolled airspace should continuously monitor . . . . . and 121.5 MHZ, when practical and not in communication on the MF or ATF frequency.
Why this is the answer
Per the TC AIM, VFR aircraft en route in uncontrolled airspace should monitor 126.7 MHz and, when practical, 121.5 MHz when not working an MF/ATF. 126.7 MHz is the en-route broadcast frequency.
Reference
TC AIM-RAC – Monitoring, Broadcasting on 126.7 MHz
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.