The technique of copying a process into main memory from secondary memory,…

Concept: Demand paging is a virtual-memory technique in which the pages of a process are not all loaded into main memory when it starts. Instead, each page is copied into main memory from secondary storage (the disk/swap area) only at the moment it is actually referenced and found to be missing there.

Application: The stem describes exactly this behaviour — content is copied into main memory from secondary memory purely on the basis of requirement. When the CPU references a page that is not yet resident (its valid/invalid bit is marked invalid), the operating system raises a page fault, locates that page on the backing store, and copies it into a free frame in main memory before execution resumes.

Cross-check — why the other options do not fit:

• Paging describes how memory is divided into fixed-size frames and pages for address translation; on its own it does not say that a page is copied in only when referenced, so plain paging does not capture the on-demand trigger the stem describes.

• Segmentation organises a process into variable-sized logical segments such as code, data, and stack; it is about how the address space is structured, not about a requirement-triggered copy between memory levels.

• Threads are units of execution scheduled within a process; they do not perform or describe any copying of data between main memory and secondary storage.

• Swapping moves an entire process (not a single page) between main memory and the backing store, usually to free memory for other processes; because it works at a different granularity (whole process, not page) and on a different trigger, it is a distinct technique from the page-level, reference-driven copying the stem describes.

So the technique of copying content into main memory from secondary memory strictly on requirement, at the page level, is demand paging.