Answer
In the plane of the page, the magnetic field comes out of the page for points a and b, and goes into the page for points c and d.
a. Force down the page, toward the wire
b. Force to the left, opposite to the direction of the current
c. Force up the page, toward the wire
d. Force to the left, opposite to the direction of the current
Now use the right-hand rule to find the direction of the force on each particle.
Work Step by Step
Use the right hand rule. If you point the thumb of your right hand in the direction of the current, your fingers curl around the wire in a way that indicates the direction of the magnetic field lines.