Final answer:
A page fault occurs when a program tries to access data that is not in physical memory, resulting in the operating system fetching the data from storage into RAM.
Step-by-step explanation:
A page fault occurs when a running program tries to access a piece of data that is not currently in physical memory (RAM). Specifically, the correct answer to the question is option b) a page cannot be accessed due to its absence from memory. When the operating system recognizes this event, it halts the program, locates the data on a storage device such as a hard disk or SSD, and transfers the data into physical memory. This procedure allows the program to access the data as if it were always in memory, although accessing it from slower storage is more time-consuming than if it had been in RAM.