An electron is projected with uniform velocity along the axis of a current carrying long solenoid. Which of the following is true?
a. The electron will be accelerated along the axis.
b. The electron path will be circular about the axis.
c. The electron will experience a force at $45^\circ$ to the axis and hence execute a helical path.
d. The electron will continue to move with uniform velocity along the axis of the solenoid.

The magnetic field inside a long current-carrying solenoid is uniform and directed along its axis.

The electron is projected with a uniform velocity along the axis of the solenoid, which means its velocity vector $\vec{v}$ is parallel or anti-parallel to the magnetic field vector $\vec{B}$.

The magnetic force experienced by a moving charge is given by $\vec{F} = q(\vec{v} \times \vec{B})$.

Since the angle $\theta$ between $\vec{v}$ and $\vec{B}$ is either $0^\circ$ or $180^\circ$, the cross product $\vec{v} \times \vec{B} = vB\sin\theta = 0$.

Therefore, the magnetic force acting on the electron is zero. In the absence of any force, the electron will not accelerate and will continue to move with its initial uniform velocity along the axis of the solenoid.

Answer: d