Runways and taxiways or portions thereof that are closed to aircraft are marked by
Why this is the answer
Closed surfaces are marked with an 'X': white Xs on closed runways, yellow Xs on closed taxiways (TC AIM-AGA, Closed Markings). The marking colour tracks the surface type.
Reference
TC AIM-AGA – Closed Markings
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.