I'm quite stumpted on this question, What I've learned is that friction must be opposing the direction of motion so that it can produce a torque which allows the object to roll without slipping. However this mark scheme implies the opposite. Also what would be the answer to the magnitude of the total force the ball exerts on the plane?
Keep in mind there is static friction and dynamic friction.
Dynamic friction is when the ball slides over the plane. Friction acts in the direction opposite the motion.
Static friction acts when there is no slipping. It keeps motion from starting. If there is a force trying to make the ball slide, static friction opposes that force.
The static friction force acts up the slope ( on the FBD ball). You can see this by applying $$ \circlearrowright^+ \sum \tau = I_{cm} \dot \omega $$ and $$ \searrow^+ \sum F = ma_{cm} $$
So think Newtons Third Law for the forces acting on the FBD of the slope.
Also, if that diagram is supposed to be the reaction of the forces on the plane from ball there should be no “weight force” and the "reaction force", so if that is the official result I think it’s flawed.
