Final answer:
When best fit, first fit, or worst fit are used as the memory allocation algorithm, external fragmentation may or may not occur.
Step-by-step explanation:
External fragmentation occurs when free memory blocks are scattered throughout the memory space, making it difficult to allocate contiguous memory for a new process. However, when best fit is used as the memory allocation algorithm, the OS tries to allocate the smallest available free block that is still larger than the requested size. This helps to minimize external fragmentation by reducing the number of smaller free blocks.
On the other hand, first fit and worst fit approaches do not guarantee the optimal allocation and can result in larger free blocks, which can increase external fragmentation.
Therefore, the correct answer is a) First fit is used, b) Best fit is used, and c) Worst fit is used.