Final answer:
The velocity selector is a device that uses both an electric field and a magnetic field to sort charged particles based on their velocity. Positively charged particles moving vertically upward would be unaffected by the vertical electric field.
Step-by-step explanation:
The subject of this question is Physics.
The velocity selector is a device that uses both an electric field and a magnetic field to sort charged particles based on their velocity. The electric field causes a force on the particles in the vertical direction, while the magnetic field provides a force perpendicular to the electric field. Only particles with the correct velocity will experience forces that cancel out, allowing them to pass through the selector undeflected.
In this case, the electric field is directed vertically upward, so positively charged particles moving vertically upward would be unaffected by the electric field. Therefore, the correct option would be (a) Positively charged particles moving vertically upward.