A very long straight wire carries a current I. At the instant when a charge + Q at point P has velocity , as shown, the force on the charge is :

The force on a moving charge in a magnetic field is given by the Lorentz force equation, $F=Q(E+v\times B)$. Since there's no electric field mentioned, we only consider the magnetic part. The magnetic field around a long straight wire carrying a current $I$ is given by $B=\frac{{\mu }_{0}I}{2\pi r}$, where $r$ is the distance from the wire. The direction of $B$ is tangential to a circle centered on the wire and can be determined by the right-hand rule. For a charge moving parallel to the wire (as implied by the velocity being in the plane of the wire and the charge), the force due to the magnetic field will be perpendicular to both the velocity of the charge and the magnetic field. Given the charge is positive and moving as shown, and assuming the current in the wire is flowing in the positive $z$ -direction (out of the page), the magnetic field at point P would be directed into the page (using the right-hand rule). The cross product $v\times B$ for a charge moving to the right (as implied) would result in a force directed along $oy$ (since $v$ is along $ox$ and $B$ is into the page). Therefore, the force on the charge is along $oy$.