Final answer:
The common shape of a partial vacuum discharge tube is cylindrical, which allows for an even distribution of electric fields and particle flow along the axis of the tube.
Step-by-step explanation:
The common shape of a partial vacuum discharge tube is C) Cylindrical. This shape is particularly useful for such devices because it allows for an even distribution of electric fields and the flow of electrons or ions along the tube's axis. In the context of different shapes and their properties in various fields of study, cylindrical symmetry is often associated with objects that have a consistent profile along the length of a central axis, much like a partial vacuum discharge tube.