Positively charged particles are projected into a magnetic field. If the direction of magnetic field is along the direction of motion of the charged particles, the particles will experience

no change in velocity

The correct answer is Option (4) → no change in velocity

The magnetic force on a charged particle is given by: $F = q (\vec{v} \times \vec{B})$

If the velocity $\vec{v}$ is parallel to the magnetic field $\vec{B}$, the cross product is zero: $\vec{v} \times \vec{B} = 0$

Therefore, the magnetic force $F = 0$, and the particles will continue to move in a straight line without deflection.

Answer: No force; particles continue in a straight line and without any change in velocity.