After reaching a certain velocity and the belt continues to move at the constant velocity(to the right).
Let's select the correct free body diagram for the box.
When the belt was at rest and the belt was placed, the horizontal velocity was zero.
When the belt started moving with some acceleration, the force of friction will try moving the box to the opposite direction.
But in this case, the belt has attained a certain velocity which makes it to start moving at a constant velocity. At constant velocity acceleration iz zero.
Since it is moving at a constant velocity, the net force is zero since there is no acceleration.
Therefore, there will be no acceleration, there is zero net force.
The free body diagram will be:
Therefore, the correct free body diagram is option b.
ANSWER:
B.