You advise ATC that you are on the downwind leg. If there is other traffic in the circuit, ATC will then
Why this is the answer
Per the TC AIM, when you report downwind with other traffic in the circuit, the tower replies with your number in the landing sequence or other appropriate instructions, so you can space yourself behind preceding traffic.
Reference
TC AIM-RAC – Initial Clearance
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.