Chuck N0IC is correct that lower frequencies are better for picking a station out of the noise with the human ear. I remember a graph in a radar book that showed the human ear was most sensitive and selective at 300Hz and the effective bandwidth rolled off gradually on either side. It has been my experience that 300Hz is kind of low and I favor something around 500Hz for actual operating. A lot of people have been exposed to 400Hz generators and their ears have been numbed to that frequency!
But for a code test the frequency should not matter if your mind is just comparing combinations of long and short beeps of some frequency.
73, Paul, AA1LL