Answer
The force is maximum when the electric current is perpendicular to the magnetic field.
Work Step by Step
This is discussed on pages 460-461.
In a later physics course, you will learn that the magnetic force is proportional to the sine of the angle between the current's direction and the magnetic field.
The sine of $90^{\circ}$ is 1, which gives the strongest force. The sine of $0^{\circ}$ is 0, which gives no force.