An electron travels along the positive x -axis. A magnetic field is...

The initial velocity of the electron is $\vec{v} = v\hat{i}$.

To reverse the direction of the electron such that it moves parallel to the negative $x$-axis, it must complete a semi-circular path. This requires a uniform magnetic field perpendicular to the initial velocity.

Since the initial velocity is along the $x$-axis, the magnetic field must lie in the $y$-$z$ plane.

If the magnetic field is directed along the $y$-axis ($\vec{B} = B\hat{j}$), the magnetic force is $\vec{F} = -e(v\hat{i} \times B\hat{j}) = -evB\hat{k}$. The electron moves in a semi-circle in the $x$-$z$ plane, and its direction is reversed.

If the magnetic field is directed along the $z$-axis ($\vec{B} = B\hat{k}$), the magnetic force is $\vec{F} = -e(v\hat{i} \times B\hat{k}) = evB\hat{j}$. The electron moves in a semi-circle in the $x$-$y$ plane, and its direction is reversed.

Thus, the magnetic field can be along the $y$-axis or the $z$-axis. The options stating "only" are incorrect.