Final answer:
An address space identifier uniquely identifies and protects a process's address space. It is vital for maintaining system stability and security by preventing memory interference.
Step-by-step explanation:
An address space identifier uniquely identifies processes and is used to provide address space protection for that process. The correct answer to the student's question is Address space identifier.
In computer systems, each process is provided with its own virtual address space to prevent one process from interfering with the memory of another process. This isolation is crucial for operating system stability and security. The system's memory manager uses the address space identifier to distinguish between different processes' address spaces, ensuring that each process can only access its own allocated memory.