Final answer:
Overlays enable a process to run in a system with limited memory by allowing the process to be larger than the available memory and by keeping only the necessary parts of the process in the memory at any given time.
Step-by-step explanation:
The idea of overlays is a strategy used in computing, particularly in the context of systems with limited memory resources. An overlay is a block of code or data loaded into the system's memory, replacing what was previously there when it is no longer needed. This allows for the execution of programs that may in fact be larger than the total amount of memory available.
With regards to the options provided:
- b) Enable a process to be larger than the amount of memory allocated to it: This is a primary function of overlays. They allow a larger program to run on a computer with limited memory by ensuring that only necessary parts of the program are loaded at any given time.
- c) Keep in memory only those instructions and data that are needed at any given time: Overlays are designed to optimize memory usage by only keeping the necessary data in memory which helps in resource management.
The other option listed, 'Enable multiple processes to execute at once', is not the purpose of overlays. This aspect is more closely related to multitasking or multithreading, which are different concepts in computer science.
Thus, the correct answers to the question are 'b' and 'c'.