Final answer:
The absence of magnetic force on a charged particle in a magnetic field B suggests the particle's velocity u is parallel or anti-parallel to B, confirming option d as the correct answer.
Step-by-step explanation:
When a particle of charge Q moving with velocity u experiences no magnetic force in a magnetic field B, this indicates that the velocity of the particle is either parallel or anti-parallel to the magnetic field. According to the cross-product of vectors which is expressed in the equation F = qvB sin θ, where θ is the angle between the velocity and the magnetic field, a zero magnetic force means that θ is either 0° or 180°. Therefore, the correct answer is d. The particle of charge is moving in parallel with the magnetic field B.
The magnetic force on a charged particle is always perpendicular to the plane formed by the velocity and the magnetic field. When this angle θ is 0° or 180°, the sine function sin θ equals 0, resulting in zero magnetic force, which matches our scenario. The magnetic field is still present even though the charged particle does not experience a magnetic force; therefore, answers a and c are incorrect. Answer b, which suggests the particle's velocity is perpendicular to the magnetic field, would result in a non-zero magnetic force, contrary to the given scenario.