This force will cause the path of the particle to curve. Therefore, at a later time, the direction of the force wil At a given moment the particle is moving in the +x direction (and the magnetic field is always in the +z direction). If q is positive, what is the direction of the force on the particle due to the magnetic field?

force is  -y direction

Explanation:

The magnetic force is given by the equation

        F = q v x B

where bold letters indicate vectors.

The direction of this force can be found by the right-hand rule, where the thumb points in the direction of speed, the other fingers extend in the direction of the magnetic field and the palm is in the direction of force for a positive charge

let's apply this to our case

the speed is in the direction + x the thumb

the magnetic field in the + z direction fingers extended

the palm is in the direction - y

force is  -y direction

the entire statement is true and correct.  

there's no diagram, nothing is illustrated, and no question is asked.