A charged particle travels along a magnetic field line. The...

The magnetic force acting on a charged particle moving in a magnetic field is given by $\vec{F} = q(\vec{v} \times \vec{B})$.

The magnitude of this force is $F = qvB\sin\theta$, where $\theta$ is the angle between the velocity vector $\vec{v}$ and the magnetic field vector $\vec{B}$.

Since the particle is traveling along a magnetic field line, its velocity is either parallel ($\theta = 0^{{}^{{}^{\circ}}}$) or anti-parallel ($\theta = 180^{{}^{{}^{\circ}}}$) to the magnetic field.

In both cases, $\sin\theta = 0$.

Therefore, the magnetic force acting on the particle is zero.