Final answer:
External fragmentation happens when there is enough total memory to satisfy a request, but it's not in one contiguous block, preventing allocation.
Step-by-step explanation:
External fragmentation occurs when a) Enough total memory exists to satisfy a request but it is not contiguous. This is a situation in memory management where free memory is separated into small blocks and is scattered throughout the system, preventing the allocation of memory despite there being enough overall. It is a common issue in systems that use fixed partition or variable partition strategies for memory allocation.
To further illustrate, assume a memory is like a row of houses. If a large family wants to move in, they need a number of houses next to each other. Even if there are enough empty houses for them, if these houses aren't all in a row (contiguous), they can't move in. This represents external fragmentation, where the total memory available is sufficient, but not in a single continuous block.
.