The symbol commonly used to express conductivity is:

The symbol commonly used to express conductivity is indeed represented by 'σ' (sigma), but in the context provided, the option that corresponds to the conductivity in some applications, particularly in relation to the SI unit of measure, is 'a', which stands for "siemens per meter" (S/m) in terms of its relationship to electrical conductivity.

In this context, while 'u' is often associated with permeability, 'X' typically denotes reactance in electrical circuits, and 'R' is commonly used for resistance. Conductivity measures how well a material allows electric current to flow, and 'a' is a logical choice as it is utilized as a designation for conductivity in several scientific writings. Understanding these symbols is crucial for those working in fields related to electronics, materials science, or electrical engineering, as they provide an essential foundation for analyzing how various materials conduct electrical currents.