At an instant of time, a magnetic field is applied along the velocity…

The correct answer is Option (4) → Zero.

The magnetic force on a moving charge is \( \vec{F} = q\,\vec{v}\times\vec{B} \). When \( \vec{B} \) is along \( \vec{v} \) the angle between them is \( 0^\circ \) (or \( 180^\circ \)), so \( \sin\theta = 0 \).

    \( \displaystyle F = qvB\sin 0^\circ = 0 \)

With no force, the acceleration is zero; the particle keeps moving along the field in a straight line.

Option (1) along the velocity. — Incorrect. A magnetic force never acts along the velocity.

Option (2) Opposite to the direction of velocity. — Incorrect. The force is zero here, so there is no retardation.

Option (3) Perpendicular to the direction of velocity. — Incorrect. The force is perpendicular to \( \vec{v} \) only when \( \vec{v} \) has a component across \( \vec{B} \).

Option (4) Zero. — Correct. The force, and hence the acceleration, is zero.

NCERT (2026-27):
Magnetic Field, Lorentz Force – “… (ii) The magnetic force \( q\,[\mathbf{v} \times \mathbf{B}] \) includes a vector product of velocity and magnetic field. The vector product makes the force due to magnetic field vanish (become zero) if velocity and magnetic field are parallel or anti-parallel.”