Horizontal momentum flux in the surface layer
Instead of just giving you the answer to a textbook problem (which would make SE a version of Chegg that future students would use to cheat on homework), I'll guide you in getting an answer.

Which variable does $\overline{u'w'}$ affect most?
For that variable, identify the surface layer.
Imagine that you are on the line. If you go straight up (that is, $w'&gt;0$) what would the difference be between the $M$ traced out and the graphed $M$? What is the sign of $w' \times u'$?
Do the same as number 3, except start at the top and go down ($w'&lt;0$).