Final answer:
The diameter of the path followed by the alpha particle is the diameter.
Step-by-step explanation:
The diameter of the path followed by the alpha particle is d) Diameter. When an alpha particle moves in a circular path due to a magnetic field, its path can be considered as a circle. The diameter of a circle is the distance between any two points on the circle passing through its center. Therefore, the diameter represents the distance that the alpha particle travels along its path.