The correct option is C Both (a) and (b)
The direction of friction will be dictated by direction of slipping of vehicle, since friction will act in a direction to oppose the slipping.
We know the equation for circular motion:
Fcentripetal=mv2r
If speed of vehicle( v) is very high, vehicle will tend to slip outwards. Since, to sustain a large speed, vehicle needs to move on a larger radius. So the frictional force will act down the incline to stop slipping.
However, if speed of vehicle( v) is on lower side, vehicle tends to slip down the incline. Hence, frictional force will be acting up the plane, to prevent slipping.
∴ option (c) is correct.