Motion of a charged particle having velocity v in a uniform magnetic field B is circular, if v is

perpendicular to B

The correct answer is Option (3) → perpendicular to B

The magnetic force is $F = q(\vec{v} \times \vec{B})$

If $\vec{v} \perp \vec{B}$, force is maximum and acts as centripetal force → circular motion.

If $\vec{v} \parallel \vec{B}$ or $\vec{v} \; \text{anti-parallel to} \; \vec{B}$, then $F=0$ → straight motion.

If $\vec{v}$ at any other angle to $\vec{B}$, motion is helical.

Hence, motion is circular if:

${\text{perpendicular to B}}$