Answer
The average rate of transporting energy is $~~4~P_{avg,1}$
Work Step by Step
When $\phi = 0$, the waves are in phase and there is fully constructive interference.
When the waves are in phase, the resultant wave has an amplitude of $2~s_m$
We know that the power $P \propto s_m^2$
Therefore, the average rate of transporting energy is $~~4~P_{avg,1}$
