If you make 2800rpm at SL, S+L with 90% power, then at 8500' you'll probably be needing more than 60% power to acheive 2800rpm when S+L. You'll need to be in a dive to achieve it, in which case both A and B are correct.
The other thing that compounds this question is that propellor geometry is optimised for a fixed TAS/RPM ratio, but combined with the airframe drag power-v-TAS requirements, this yields only one optimum point (at least when S+L). If you move away from this point, efficiency is degraded. It might be that at SL, the efficiency is worse than at 8500'