Final answer:
The element that allows z/OS systems to share PARMLIB definitions with unique values is System Symbols. These symbols enable customization of shared configurations, by assigning unique values to symbolic names for each system.
Step-by-step explanation:
The correct answer to the question is C. System Symbols. In z/OS, system symbols are used to allow different systems to share PARMLIB definitions while retaining unique values for each system. System symbols are symbolic names that can be used in PARMLIB members and other system definitions to represent site-specific values. Each system can define its own symbols with unique values, enabling a shared PARMLIB structure with customization for individual systems. This mechanism provides flexibility and helps to manage complex environments with multiple z/OS systems.
For example, if you have a symbol defined as SYSNAME containing the unique system name, it can be used across various PARMLIB members. When each z/OS system references the symbol SYSNAME, it will be substituted with the system-specific name that has been assigned to it, thus allowing for shared yet individualized configuration.